Add RHEL8 Universal Base Image with platform-python to CI unit tests (#21655)

.github/workflows/linux_unit_tests.yaml

@@ -103,6 +103,7 @@ jobs:
     - uses: codecov/codecov-action@v1
       with:
         flags: shelltests,linux
+
   centos6:
     # Test for Python2.6 run on Centos 6
     runs-on: ubuntu-latest
@@ -117,6 +118,32 @@ jobs:
           git fetch origin ${{ github.ref }}:test-branch
           git checkout test-branch
           share/spack/qa/run-unit-tests
+
+  rhel8-platform-python:
+    runs-on: ubuntu-latest
+    container: registry.access.redhat.com/ubi8/ubi
+    steps:
+    - name: Install dependencies
+      run: |
+          dnf install -y \
+              bzip2 curl file gcc-c++ gcc gcc-gfortran git gnupg2 gzip \
+              make patch tcl unzip which xz
+    - uses: actions/checkout@v2
+    - name: Setup repo and non-root user
+      run: |
+          git --version
+          git fetch --unshallow
+          . .github/workflows/setup_git.sh
+          useradd spack-test
+          chown -R spack-test .
+    - name: Run unit tests
+      env:
+          SPACK_PYTHON: /usr/libexec/platform-python
+      shell: runuser -u spack-test -- bash {0}
+      run: |
+          source share/spack/setup-env.sh
+          spack unit-test -k 'not svn and not hg' -x --verbose
+
   clingo:
     # Test for the clingo based solver
     runs-on: ubuntu-latest