contrib/cirrus/prebuild.sh: drop unused get_env_key

The last user of get_env_key was removed by commit
69163af2a0 ("Remove gitleaks scanning").

Remove it.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>