Stop paying the unreachable cache server's timeout

setup-go caches by default. act_runner's built-in cache server binds to
an address job containers cannot route to, so every restore and save
blocks until it times out:

  Failed to restore: getCacheEntry failed: connect ETIMEDOUT 10.0.2.12:40789
  Failed to save:    reserveCache failed: connect ETIMEDOUT 10.0.2.12:40789

That was 634s of a 946s build. Disable caching until the runner's
cache.host points somewhere reachable, then turn it back on.

.gitea/workflows/build.yml

@@ -18,7 +18,11 @@ jobs:
       - uses: actions/setup-go@v5
         with:
           go-version: ${{ env.GO_VERSION }}
-          cache-dependency-path: go.sum
+          # setup-go caches by default, and this runner's built-in cache server
+          # is unreachable from job containers -- every restore and save hangs
+          # until it times out, which cost 634s of a 946s build. Re-enable once
+          # act_runner's cache.host points somewhere job containers can route.
+          cache: false
 
       - name: Format
         run: test -z "$(gofmt -l .)" || { gofmt -l .; echo "run gofmt -w ."; exit 1; }