Cryptic error message when having multiple gpg keys

[ghostbuster91]

If user has multiple keys and the imported one isn't the default one (which is often the case in my opinion) the signing process will failed with not very helpful message:

[info] gpg: signing failed: Bad passphrase
[info] gpg: signing failed: Bad passphrase
[error] stack trace is suppressed; run last rules2_11 / signedArtifacts for the full output
[error] (rules2_11 / signedArtifacts) Failure running 'gpg --batch --pinentry-mode loopback --passphrase ... --detach-sign --armor --use-agent --output /home/kghost/workspace/scalafix-unified/rules/target/jvm-2.11/unified_2.11-0.0.2+3-29124f7a+20220206-1145-SNAPSHOT.pom.asc /home/kghost/workspace/scalafix-unified/rules/target/jvm-2.11/unified_2.11-0.0.2+3-29124f7a+20220206-1145-SNAPSHOT.pom'.  Exit code: 2
[error] Total time: 5 s, completed Feb 6, 2022, 11:45:06 AM

After carefully inspecting the gpg command we might notice that there is no information about which key to use.

sbt-pgp exposes usePgpKeyHex method to explicitly set key which will be used to perform operations. https://github.com/sbt/sbt-pgp#configuration-signing-key

I think that it should be possible to obtain key_id during the process and set it somewhere among the lines: https://github.com/sbt/sbt-ci-release/blob/main/plugin/src/main/scala/com/geirsson/CiReleasePlugin.scala#L143

Otherwise, please consider adding some note to the readme with the link to relevant section in the sbt-pgp.