BazaarDownloadStrategy: Replace in-place sub

livecheck's `Git` strategy uses `DownloadStrategyDetector#detect`
in its `#match?` method to check if a URL is a Git repository. This
has historically worked fine but I've recently seen a `can't modify
frozen String` error for a few formulae (percona-toolkit,
schroedinger, squid) in relation to the in-place `sub` call in
`BazaarDownloadStrategy`'s initializer.

Other download strategies use a `@url = @url.sub(...)` pattern to
avoid this issue, so this commit resolves the issue by using the same
approach in `BazaarDownloadStrategy`.

Library/Homebrew/download_strategy.rb

@@ -1277,7 +1277,7 @@ end
 class BazaarDownloadStrategy < VCSDownloadStrategy
   def initialize(url, name, version, **meta)
     super
-    @url.sub!(%r{^bzr://}, "")
+    @url = @url.sub(%r{^bzr://}, "")
   end
 
   # @see AbstractDownloadStrategy#source_modified_time