fix: use runs-on: ubuntu-24.04 label so forgejo-runner derives correct ImageOS

forgejo's act runner unconditionally overwrites the ImageOS env var based on
the runs-on: platform label after job env is merged (run_context.go withGithubEnv),
hardcoding ubuntu-latest to ImageOS=ubuntu20 regardless of the actual container
image or any manual env override. Since ubuntu-20.04 has no stable OTP 29.0
builds, setup-beam failed even with an explicit ImageOS env var (silently
clobbered). Using the ubuntu-24.04 runner label (already registered to the
same node:24-bookworm image) makes the runner correctly derive ImageOS=ubuntu24,
which has real stable OTP 29.0 builds. Switched version-type back to strict
since we're now targeting a platform with the exact pinned versions.

.forgejo/workflows/ci.yml

@@ -9,7 +9,7 @@ on:
 jobs:
   quality:
     name: Build and test
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-24.04
     services:
       postgres:
         image: postgis/postgis:17-3.5
@@ -24,7 +24,6 @@ jobs:
           --health-retries 5
     env:
       MIX_ENV: test
-      ImageOS: ubuntu24
       PGHOST: postgres
 
     steps:
@@ -40,7 +39,7 @@ jobs:
       - name: Set up Elixir
         uses: http://forgejo:3000/graham/setup-beam@v1
         with:
-          version-type: loose
+          version-type: strict
           elixir-version: '1.20.2'
           otp-version: '29.0'
 
@@ -82,10 +81,9 @@ jobs:
 
   dialyzer:
     name: Dialyzer
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-24.04
     env:
       MIX_ENV: dev
-      ImageOS: ubuntu24
 
     steps:
       - name: Configure git for submodules
@@ -100,7 +98,7 @@ jobs:
       - name: Set up Elixir
         uses: http://forgejo:3000/graham/setup-beam@v1
         with:
-          version-type: loose
+          version-type: strict
           elixir-version: '1.20.2'
           otp-version: '29.0'
 
@@ -127,7 +125,7 @@ jobs:
 
   build-and-push:
     name: Build and Push Docker Image
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-24.04
     needs: [quality, dialyzer]
     if: github.event_name == 'push'