Fix CI: ruby/setup-ruby doesn't recognize "file" as a version keyword

Omitting ruby-version entirely triggers its actual auto-detection from
.ruby-version — "file" was a mistaken guess and failed with "Unknown
engine file".

.gitea/workflows/ci.yml

@@ -10,9 +10,11 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      # No ruby-version input: the action auto-detects it from
+      # .ruby-version when the input is omitted (not "file", which isn't
+      # a recognized value and fails with "Unknown engine file").
       - uses: ruby/setup-ruby@v1
         with:
-          ruby-version: file # reads .ruby-version
           bundler-cache: true
 
       - name: Ruby syntax check