Replace buildenv image with a container reference

.gitea/workflows/build.yml

@@ -9,13 +9,13 @@ on:
 
 jobs:
   audit:
-    runs-on: buildenv
+    container: buildenv:5
     steps:
       - uses: actions/checkout@v4
       - run: hlint -- src tests
 
   test:
-    runs-on: buildenv
+    container: buildenv:5
     steps:
       - uses: actions/checkout@v4
       - name: Install dependencies
@@ -25,7 +25,7 @@ jobs:
       - run: cabal test --test-show-details=streaming --enable-tests
 
   doc:
-    runs-on: buildenv
+    container: buildenv:5
     steps:
       - uses: actions/checkout@v4
       - name: Install dependencies

.gitea/workflows/release.yml

@@ -7,7 +7,7 @@ on:
 
 jobs:
   release:
-    runs-on: buildenv
+    container: buildenv:5
     steps:
       - uses: actions/checkout@v4
       - name: Upload a candidate