build: anchor regex and escape dots in downloader config

Author: rickeylev

downloader_config.cfg

@@ -1,6 +1,6 @@
 # The rewrite can be removed when https://github.com/bazelbuild/platforms/issues/139 is implemented
 # Try GitHub first (primary)
-rewrite github.com/bazelbuild/platforms/(.*) github.com/bazelbuild/platforms/$1
+rewrite ^https?://github\.com/bazelbuild/platforms/(.*) https://github.com/bazelbuild/platforms/$1
 # Fall back to mirror (secondary)
 # Tracking upstream BCR mirror addition: https://github.com/bazelbuild/platforms/issues/139
-rewrite github.com/bazelbuild/platforms/(.*) mirror.bazel.build/github.com/bazelbuild/platforms/$1
+rewrite ^https?://github\.com/bazelbuild/platforms/(.*) https://mirror.bazel.build/github.com/bazelbuild/platforms/$1