Fix CI mkdocs related jobs not to use env

There are 2 issues:

1. The `env` variable being used doesn't exist.
2. Using `env` is not supported in `runs-on`.

This fix was done in the past but was not ported to the branch that
implemented `mkdocs`, so it slipped in again.

Signed-off-by: Leandro Lucarella <[email protected]>
#
# On branch fix-ci
# Changes to be committed:
#	modified:   cookiecutter/{{cookiecutter.github_repo_name}}/.github/workflows/ci.yaml
#
# Untracked files:
#	frequenz-repo-config-python/
#	src/frequenz/repo-config/
#	tests/test_repo-config.py
#

Author: llucax

.github/workflows/ci.yaml

@@ -86,7 +86,7 @@ jobs:
   test-docs:
     name: Test documentation website generation
     if: github.event_name != 'push'
-    runs-on: ubuntu-${{ env.DEFAULT_UBUNTU_VERSION }}
+    runs-on: ubuntu-20.04
     steps:
       - name: Fetch sources
         uses: actions/checkout@v3
@@ -123,7 +123,7 @@ jobs:
     name: Publish documentation website to GitHub pages
     needs: ["test", "build"]
     if: github.event_name == 'push'
-    runs-on: ubuntu-${{ env.DEFAULT_UBUNTU_VERSION }}
+    runs-on: ubuntu-20.04
     permissions:
       contents: write
     steps:

cookiecutter/{{cookiecutter.github_repo_name}}/.github/workflows/ci.yaml

@@ -86,7 +86,7 @@ jobs:
   test-docs:
     name: Test documentation website generation
     if: github.event_name != 'push'
-    runs-on: ubuntu-{{'${{ env.DEFAULT_UBUNTU_VERSION }}'}}
+    runs-on: ubuntu-20.04
     steps:
       - name: Fetch sources
         uses: actions/checkout@v3
@@ -123,7 +123,7 @@ jobs:
     name: Publish documentation website to GitHub pages
     needs: ["test", "build"]
     if: github.event_name == 'push'
-    runs-on: ubuntu-{{'${{ env.DEFAULT_UBUNTU_VERSION }}'}}
+    runs-on: ubuntu-20.04
     permissions:
       contents: write
     steps: