Move signing.secretKeyRingFile from gradle.properties into build.gradle

This allows correctly setting it to an absolute path based on the
project directory. The location of the secret key ring is not sensitive
information, so it does not need to be stored encrypted.
4 jobs for develop in 3 minutes and 18 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Test
passed tests #132581

00:01:26

passed tests:jdk-11 #132582

00:01:22

passed tests:jdk-14 #132583

00:02:00

 
  Deploy
failed publish #132584

00:01:17

 
Name Stage Failure
failed
publish Deploy


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BUILD FAILED in 43s
18 actionable tasks: 18 executed
Running after_script
Uploading artifacts for failed job
ERROR: Job failed: exit code 1