Reorder workflow execution in GHA (#32183)

This patchset refactors our GitHub actions into a single top-level ci workflow that
invokes a series of reusable actions.  The main goal of this is to be able to easily
control which tests run and in what order based on the success or failure of top-level
prechecks.  Our previous workflows ran in three sets:

* nix tests: style and verification first, then linux and macos tests if successful
* windows tests: style and verification first, then linux and macos tests if successful
* bootstrap tests

As a result, the bootstrap tests ran even if the style failed, and style and verification
had to run on two different platforms despite running identical checks.  I'm relatively
sure that's because of the limitation on dependencies between steps in the jobs.
Reusable workflows allow us to run the style, verification and now audit checks once,
then depending on the results, and the files changed, run the appropriate nix, windows
and bootstrap tests.  While it saves only a few minutes by itself, this makes it easier to
refactor checks to subset tests without having to replicate tests or other workflow
components in the future.

Co-authored-by: Massimiliano Culpo <massimiliano.culpo@gmail.com>

.github/workflows/windows_python.yml

@@ -1,17 +1,10 @@
-name: windows tests
+name: windows
 
 on:
-  push:
-    branches:
-      - develop
-      - releases/**
-  pull_request:
-    branches:
-      - develop
-      - releases/**
+  workflow_call:
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.run_number }}
+  group: windows-${{ github.workflow }}-${{ github.event.pull_request.number || github.run_number }}
   cancel-in-progress: true
 
 defaults:
@@ -19,45 +12,7 @@ defaults:
     shell:
      powershell Invoke-Expression -Command ".\share\spack\qa\windows_test_setup.ps1"; {0}
 jobs:
-  validate:
-    runs-on: windows-latest
-    steps:
-    - uses: actions/checkout@2541b1294d2704b0964813337f33b291d3f8596b
-    - uses: actions/setup-python@b55428b1882923874294fa556849718a1d7f2ca5
-      with:
-        python-version: 3.9
-    - name: Install Python Packages
-      run: |
-        python -m pip install --upgrade pip
-        python -m pip install --upgrade vermin
-    - name: vermin (Spack's Core)
-      run: vermin --backport argparse --backport typing -t='2.7-' -t='3.6-' -v spack/lib/spack/spack/ spack/lib/spack/llnl/ spack/bin/
-    - name: vermin (Repositories)
-      run: vermin --backport argparse --backport typing -t='2.7-' -t='3.6-' -v spack/var/spack/repos
-  # Run style checks on the files that have been changed
-  style:
-    runs-on: windows-latest
-    steps:
-    - uses: actions/checkout@2541b1294d2704b0964813337f33b291d3f8596b
-      with:
-        fetch-depth: 0
-    - uses: actions/setup-python@b55428b1882923874294fa556849718a1d7f2ca5
-      with:
-        python-version: 3.9
-    - name: Install Python packages
-      run: |
-        python -m pip install --upgrade pip six setuptools flake8 "isort>=4.3.5" "mypy>=0.800" "click==8.0.4" "black<=21.12b0" pywin32 types-python-dateutil
-    - name: Create local develop
-      run: |
-        .\spack\.github\workflows\setup_git.ps1
-    - name: Run style tests
-      run: |
-        spack style
-    - name: Verify license headers
-      run: |
-        python spack\bin\spack license verify
-  unittest:
-    needs: [ validate, style ]
+  unit-tests:
     runs-on: windows-latest
     steps:
     - uses: actions/checkout@2541b1294d2704b0964813337f33b291d3f8596b
@@ -76,8 +31,7 @@ jobs:
       run: |
         echo F|xcopy .\spack\share\spack\qa\configuration\windows_config.yaml $env:USERPROFILE\.spack\windows\config.yaml
         spack unit-test --verbose --ignore=lib/spack/spack/test/cmd
-  unittest-cmd:
-    needs: [ validate, style ]
+  unit-tests-cmd:
     runs-on: windows-latest
     steps:
     - uses: actions/checkout@2541b1294d2704b0964813337f33b291d3f8596b
@@ -96,8 +50,7 @@ jobs:
       run: |
         echo F|xcopy .\spack\share\spack\qa\configuration\windows_config.yaml $env:USERPROFILE\.spack\windows\config.yaml
         spack unit-test lib/spack/spack/test/cmd --verbose
-  buildtest:
-    needs: [ validate, style ]
+  build-abseil:
     runs-on: windows-latest
     steps:
     - uses: actions/checkout@2541b1294d2704b0964813337f33b291d3f8596b
@@ -116,8 +69,7 @@ jobs:
         spack external find cmake
         spack external find ninja
         spack install abseil-cpp
-  generate-installer-test:
-    needs: [ validate, style ]
+  make-installer:
     runs-on: windows-latest
     steps:
     - name: Disable Windows Symlinks
@@ -153,7 +105,7 @@ jobs:
         name: Windows Spack Installer
         path: ${{ env.installer_root}}\pkg\Spack.msi
   execute-installer:
-    needs: generate-installer-test
+    needs: make-installer
     runs-on: windows-latest
     defaults:
       run: