fix: use in-memory PGP signing instead of writing key to tmp file

[didiergarcia]

Summary

• Replaces the approach of writing the base64-decoded PGP key to a tmp file (tmp/key.pgp) with setting it as an in-memory string property (signing.key)
• mvn-publish.gradle already uses useInMemoryPgpKeys() which reads from signing.key — the old file-based approach was incompatible with it
• Removes the tmp directory creation side effect during builds

Test plan

• Verified working end-to-end: the 1.26.0 release was published to Maven Central successfully with this change in place