Temporarily pin Ubuntu to v22.04, where we use kcov (#48152)

Ubuntu doesn't package kcov in v24.04 Since GitHub
started upgrading their runner images, this makes
our CI fail, see e.g.

https://github.com/spack/spack/actions/runs/12366970840/job/34518012887?pr=47854

This is a temporary workaround, while we prepare a
more stable fix.

* Don't run too many unit tests

.github/workflows/unit_tests.yaml

@@ -14,14 +14,14 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-latest]
+        os: [ubuntu-22.04]
         python-version: ['3.7', '3.8', '3.9', '3.10', '3.11', '3.12']
         concretizer: ['clingo']
         on_develop:
         - ${{ github.ref == 'refs/heads/develop' }}
         include:
         - python-version: '3.11'
-          os: ubuntu-latest
+          os: ubuntu-20.04
           concretizer: original
           on_develop: ${{ github.ref == 'refs/heads/develop' }}
         - python-version: '3.6'
@@ -30,24 +30,24 @@ jobs:
           on_develop: ${{ github.ref == 'refs/heads/develop' }}
         exclude:
         - python-version: '3.7'
-          os: ubuntu-latest
           concretizer: 'clingo'
+          os: ubuntu-22.04
           on_develop: false
         - python-version: '3.8'
-          os: ubuntu-latest
           concretizer: 'clingo'
+          os: ubuntu-22.04
           on_develop: false
         - python-version: '3.9'
-          os: ubuntu-latest
           concretizer: 'clingo'
+          os: ubuntu-22.04
           on_develop: false
         - python-version: '3.10'
-          os: ubuntu-latest
           concretizer: 'clingo'
+          os: ubuntu-22.04
           on_develop: false
         - python-version: '3.11'
-          os: ubuntu-latest
           concretizer: 'clingo'
+          os: ubuntu-22.04
           on_develop: false
 
     steps:
@@ -98,7 +98,7 @@ jobs:
         verbose: true
   # Test shell integration
   shell:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     steps:
     - uses: actions/checkout@0ad4b8fadaa221de15dcec353f45205ec38ea70b
       with:
@@ -158,7 +158,7 @@ jobs:
           spack unit-test -k 'not cvs and not svn and not hg' -x --verbose
   # Test for the clingo based solver (using clingo-cffi)
   clingo-cffi:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     steps:
     - uses: actions/checkout@0ad4b8fadaa221de15dcec353f45205ec38ea70b
       with: