testing: unreliable `gpg --recv-keys` command

[tmatsuo]

In .kokoro/docker/Dockerfile, we verify Python source code tarball with gpg command. gpg --recv-keys often fails to fetch some keys. In that case the command itself returns 0, but after all the script will fail when it tries to verify the corresponding Python tarball.

Maybe we can introduce another script for retrying the given command few times and receive the key one by one.

[tmatsuo]

Happened on #4160

[tmatsuo]

Or maybe we can have the public keys in the repo.