ci: make runner labels configurable via CI_RUNNERS variable

Switch all workflow jobs from a hardcoded ubuntu-latest to
${{ vars.CI_RUNNERS || 'ubuntu-latest' }}, so runner sizing can be
changed via the CI_RUNNERS organization/repository variable without
editing workflow files.

Signed-off-by: Marco Nenciarini <marco.nenciarini@enterprisedb.com>

.github/workflows/ci.yml

@@ -8,7 +8,7 @@ permissions: read-all
 
 jobs:
   ci:
-    runs-on: ubuntu-latest
+    runs-on: ${{ vars.CI_RUNNERS || 'ubuntu-latest' }}
     permissions:
       packages: write
       contents: write

.github/workflows/publish-docs.yml

@@ -20,7 +20,7 @@ permissions: read-all
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ${{ vars.CI_RUNNERS || 'ubuntu-latest' }}
     permissions:
       contents: read
     steps:
@@ -60,7 +60,7 @@ jobs:
       name: github-pages
       url: ${{ steps.deployment.outputs.page_url }}
     needs: build
-    runs-on: ubuntu-latest
+    runs-on: ${{ vars.CI_RUNNERS || 'ubuntu-latest' }}
     steps:
       - name: Deploy to GitHub Pages
         id: deployment

.github/workflows/release-please.yml

@@ -9,7 +9,7 @@ permissions: read-all
 
 jobs:
   release-please:
-    runs-on: ubuntu-latest
+    runs-on: ${{ vars.CI_RUNNERS || 'ubuntu-latest' }}
     permissions:
       contents: write
       pull-requests: write

.github/workflows/release-publish.yml

@@ -7,7 +7,7 @@ permissions: read-all
 
 jobs:
   release-publish-artifacts:
-    runs-on: ubuntu-latest
+    runs-on: ${{ vars.CI_RUNNERS || 'ubuntu-latest' }}
     permissions:
       packages: write
       contents: write