Use buildifier_test instead of 'mode = diff'

```
DEBUG: /home/runner/.cache/bazel/_bazel_runner/2f1e0776f91a8e59682a44ab965a403c/external/com_github_bazelbuild_buildtools/buildifier/internal/factory.bzl:17:10: DEPRECATION NOTICE: value 'diff' for attribute 'mode' will be removed in the future. Migrate '@//:buildifier-diff' to buildifier_test.
DEBUG: /home/runner/.cache/bazel/_bazel_runner/2f1e0776f91a8e59682a44ab965a403c/external/com_github_bazelbuild_buildtools/buildifier/internal/factory.bzl:28:10: DEPRECATION NOTICE: attribute 'diff_command' will be removed in the future. Migrate '@//:buildifier-diff' to buildifier_test.
```

Author: avdv

.github/workflows/workflow.yaml

@@ -74,7 +74,7 @@ jobs:
       - name: Test for buildifier suggestions
         uses: tweag/run-nix-shell@v0
         with:
-          run: bazel run //:buildifier-diff
+          run: bazel test //:buildifier-test
 
   all_ci_tests:
     runs-on: ubuntu-latest

BUILD.bazel

@@ -1,12 +1,13 @@
-load("@com_github_bazelbuild_buildtools//buildifier:def.bzl", "buildifier")
+load("@com_github_bazelbuild_buildtools//buildifier:def.bzl", "buildifier", "buildifier_test")
 
 exports_files(["defs.bzl"])
 
-buildifier(
-    name = "buildifier-diff",
+buildifier_test(
+    name = "buildifier-test",
     buildifier = "@buildifier//:buildifier",
     diff_command = "diff -u",
-    mode = "diff",
+    no_sandbox = True,
+    workspace = ":WORKSPACE",
 )
 
 buildifier(