Add SOURCE_TAG to show source repository's tag (mastodon#10698)

Author: ushitora-anqou

lib/mastodon/version.rb

@@ -42,7 +42,7 @@ def source_base_url
 
     # specify git tag or commit hash here
     def source_tag
-      nil
+      ENV.fetch('SOURCE_TAG') { nil }
     end
 
     def source_url