ci: update workflows to use 'linux' runner label

- Change runs-on from ubuntu-latest to linux to match runner labels
- Fixes workflow execution with thursday-macos runner

.gitea/workflows/build.yml

@@ -11,7 +11,7 @@ env:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: linux
     steps:
       - name: Checkout code
         uses: actions/checkout@v4

.gitea/workflows/pr-validation.yml

@@ -6,7 +6,7 @@ on:
 
 jobs:
   validate-pr-title:
-    runs-on: ubuntu-latest
+    runs-on: linux
     steps:
       - name: Validate PR title follows Conventional Commits
         run: |
@@ -47,7 +47,7 @@ jobs:
           fi
 
   validate-docker:
-    runs-on: ubuntu-latest
+    runs-on: linux
     steps:
       - name: Checkout code
         uses: actions/checkout@v4
@@ -84,7 +84,7 @@ jobs:
           fi
 
   build-test:
-    runs-on: ubuntu-latest
+    runs-on: linux
     steps:
       - name: Checkout code
         uses: actions/checkout@v4
@@ -143,7 +143,7 @@ jobs:
           echo "✅ All container tests passed"
 
   check-files:
-    runs-on: ubuntu-latest
+    runs-on: linux
     steps:
       - name: Checkout code
         uses: actions/checkout@v4