bin/brew: use a function to avoid issues.

Otherwise when `bin/brew` is updated in-place Bash will carry on from
the offset and this can cause issues.

Thanks to `@UniqMartin` for pointing this out.

bin/brew

@@ -156,8 +156,7 @@ then
   # Hide shellcheck complaint:
   # shellcheck source=/dev/null
   source "$HOMEBREW_BASH_COMMAND"
-  "$HOMEBREW_COMMAND" "$@"
-  exit $?
+  { "$HOMEBREW_COMMAND" "$@"; exit $?; }
 else
   exec "$HOMEBREW_RUBY_PATH" -W0 "$HOMEBREW_LIBRARY/brew.rb" "$@"
 fi