fix: Pin select jobs to ubuntu 22.04.

Using Ubuntu 24.04 breaks Mongo installation
and some thing involving lxml/xmlsec. We are
going to pin this until we're ready to upgrade both.

.github/workflows/unit-tests.yml

@@ -15,7 +15,7 @@ concurrency:
 jobs:
   run-tests:
     name: ${{ matrix.shard_name }}(py=${{ matrix.python-version }},dj=${{ matrix.django-version }},mongo=${{ matrix.mongo-version }})
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     strategy:
       matrix:
         python-version:
@@ -164,7 +164,7 @@ jobs:
           overwrite: true
 
   collect-and-verify:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     steps:
       - uses: actions/checkout@v4
       - name: Setup Python
@@ -229,7 +229,7 @@ jobs:
   # https://github.com/orgs/community/discussions/33579
   success:
     name: Unit tests successful
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     if: always()
     needs: [run-tests]
     steps:
@@ -240,7 +240,7 @@ jobs:
           jobs: ${{ toJSON(needs) }}
 
   compile-warnings-report:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     needs: [run-tests]
     steps:
       - uses: actions/checkout@v4
@@ -268,7 +268,7 @@ jobs:
           overwrite: true
 
   merge-artifacts:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     needs: [compile-warnings-report]
     steps:
       - name: Merge Pytest Warnings JSON Artifacts
@@ -288,7 +288,7 @@ jobs:
   # Combine and upload coverage reports.
   coverage:
     if: (github.repository == 'edx/edx-platform-private') || (github.repository == 'openedx/edx-platform' && (startsWith(github.base_ref, 'open-release') == false))
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     needs: [run-tests]
     strategy:
       matrix: