ci: use 'linux' runner label instead of 'ubuntu-latest'

Gitea runners use 'linux' label, not GitHub's 'ubuntu-latest'.

.gitea/workflows/ci.yml

@@ -5,7 +5,7 @@ on:
 
 jobs:
   fmt:
-    runs-on: ubuntu-latest
+    runs-on: linux
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-go@v5
@@ -20,7 +20,7 @@ jobs:
           fi
 
   test:
-    runs-on: ubuntu-latest
+    runs-on: linux
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-go@v5