ci: use Ubuntu 22.04 for GitHub actions

ubuntu-latest has recently switched to Ubuntu 24.04, with some breaking
changes, among others restricting the creation of namespaces as
unprivileged user, but also other compatibility issues within Python
itself.

Until this is resolved, we pin the runs to Ubuntu 22.04.

.github/workflows/ghcr.yml

@@ -25,7 +25,7 @@ jobs:
   # Push image to GitHub Packages.
   # See also https://docs.docker.com/build/ci/github-actions/
   container-registry:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     permissions:
       packages: write
       contents: read

.github/workflows/pypi.yml

@@ -6,7 +6,7 @@ on:
 
 jobs:
   publish:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     steps:
       - uses: actions/checkout@v4
 

.github/workflows/tests.yml

@@ -17,7 +17,7 @@ concurrency:
 jobs:
   lint:
     name: Lint
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     env:
       ENV: dev
     steps:
@@ -47,7 +47,7 @@ jobs:
   docker-tests:
     name: Docker tests
     needs: [lint]
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     env:
       ENV: dev
     steps:
@@ -66,7 +66,7 @@ jobs:
   compatibility-tests:
     name: Compatibility tests
     needs: [lint]
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     strategy:
       fail-fast: false
       matrix: