Fix: Workaround to load MASTODON_VERSION_* in Docker. (#26591)

Co-authored-by: Claire <claire.github-309c@sitedethib.com>

lib/mastodon/version.rb

@@ -17,7 +17,7 @@ module Mastodon
     end
 
     def flags
-      ENV.fetch('MASTODON_VERSION_FLAGS', '-beta2')
+      ENV['MASTODON_VERSION_FLAGS'].presence || '-beta2'
     end
 
     def suffix