Add check to keyring import for installed keys

[vadave]

I was working on some automation and discovered the keyring import function was taking a long time to run, even if the gpg keys had already been installed. Looking at the import-release-team-keyring script, I noticed that it doesn't check to see if the key is already installed locally before it queries the keyservers.

I've added a simple check in that function that will bypass the network calls if the key is already installed in the keyring. I'll submit a PR to address this.

[vadave]

PR has been merged. Closing. Thanks to the team for the great plugin.