bin/brew: allow GITHUB_ACTIONS_HOMEBREW_SELF_HOSTED.

Otherwise test-bot will not behave as expected.

bin/brew

@@ -94,7 +94,8 @@ then
   # Filter all but the specific variables.
   for VAR in HOME SHELL PATH TERM TERMINFO COLUMNS DISPLAY LOGNAME USER CI SSH_AUTH_SOCK SUDO_ASKPASS \
              http_proxy https_proxy ftp_proxy no_proxy all_proxy HTTPS_PROXY FTP_PROXY ALL_PROXY \
-             GITHUB_ACTIONS GITHUB_REPOSITORY GITHUB_RUN_ID GITHUB_SHA GITHUB_HEAD_REF GITHUB_BASE_REF GITHUB_REF \
+             GITHUB_ACTIONS GITHUB_ACTIONS_HOMEBREW_SELF_HOSTED \
+             GITHUB_REPOSITORY GITHUB_RUN_ID GITHUB_SHA GITHUB_HEAD_REF GITHUB_BASE_REF GITHUB_REF \
              "${!HOMEBREW_@}"
   do
     # Skip if variable value is empty.