CI: switch some jobs to using ubuntu-slim (#6208)

See https://github.blog/changelog/2026-01-22-1-vcpu-linux-runner-now-generally-available-in-github-actions/

Author: fingolfin

.github/workflows/lint.yml

@@ -12,7 +12,7 @@ on:
 
 jobs:
   gaplint:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-slim
     steps:
       - name: "Check out the repository"
         uses: actions/checkout@v6

.github/workflows/release.yml

@@ -33,7 +33,7 @@ env:
 jobs:
   tools:
     name: "Validate release scripts"
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-slim
     steps:
       - uses: actions/checkout@v6
 
@@ -262,7 +262,7 @@ jobs:
       - unix
       - cygwin
     if: ${{ always() && github.event_name != 'pull_request' && github.repository == 'gap-system/gap' }}
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-slim
 
     steps:
       - name: Get branch name

.github/workflows/update-gh-pages.yml

@@ -25,7 +25,7 @@ concurrency:
 jobs:
   # Build job
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-slim
     steps:
       - name: Install dependencies
         run: |
@@ -76,7 +76,7 @@ jobs:
     environment:
       name: github-pages
       url: ${{ steps.deployment.outputs.page_url }}
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-slim
     needs: build
     steps:
       - name: Deploy to GitHub Pages