Use correct integrity hash for sprockets caching

Use self.integrity_uri over self.hexdigest_integrity_uri(digest), as the
latter always returned nil leading to improper cache invalidation.

We are alos enabling caching of `tmp/cache/assets/sprockets` again, to
get those performance gains.

.gitlab-ci.yml

@@ -388,13 +388,11 @@ flaky-examples-check:
 
 .assets-compile-cache: &assets-compile-cache
   cache:
-    key: "assets-compile:vendor_ruby:.yarn-cache:tmp_cache_assets_sprockets:v4"
+    key: "assets-compile:vendor_ruby:.yarn-cache:tmp_cache_assets_sprockets:v5"
     paths:
       - vendor/ruby/
       - .yarn-cache/
-      # We have disabled caching of sprockets for now, as it fails to pick up changes in SCSS:
-      # https://gitlab.com/gitlab-org/gitlab-ce/issues/57431
-      # - tmp/cache/assets/sprockets
+      - tmp/cache/assets/sprockets
 
 compile-assets:
   <<: *dedicated-runner

lib/gitlab/patch/sprockets_base_file_digest_key.rb

@@ -9,7 +9,7 @@ module Gitlab
       def file_digest(path)
         if stat = self.stat(path)
           digest = self.stat_digest(path, stat)
-          integrity_uri = self.hexdigest_integrity_uri(digest)
+          integrity_uri = self.integrity_uri(digest)
 
           key = Sprockets::UnloadedAsset.new(path, self).file_digest_key(integrity_uri)
           cache.fetch(key) do