ci-test: Fix syntax/formatting check so it actually emits an exit code

The old format didn't emit any exit codes and thus did not cause CI to
fail:

    $ diff -u <(echo -n) <(gofmt -d -s .)
    make.go:513:46: missing ',' before newline in composite literal
    $ echo $?
    0

The smallest possible fix to this is to ensure gofmt output actually
goes to stdout as the original test expected:

    $ diff -u <(echo -n) <(gofmt -d -s . 2>&1)
    --- /dev/fd/63	2025-01-05 07:46:28.550632333 +0000
    +++ /dev/fd/62	2025-01-05 07:46:28.550632333 +0000
    @@ -0,0 +1 @@
    +make.go:513:46: missing ',' before newline in composite literal
    $ echo $?
    1

.github/workflows/ci-test.yml

@@ -23,7 +23,7 @@ jobs:
           go-version: ${{ matrix.go-version }}
           check-latest: true
 
-      - run: diff -u <(echo -n) <(gofmt -d -s .)
+      - run: diff -u <(echo -n) <(gofmt -d -s . 2>&1)
 
       - run: go install -v ./...