Update signing key environment variable (#283)

Change the property name to search for `GPG_SECRET_KEY`

## Motivation and Context
enable signing publications

## Breaking Changes
no

## Types of changes
- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update

## Checklist
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [x] New and existing tests pass locally
- [ ] I have added appropriate error handling
- [ ] I have added or updated documentation as needed

buildSrc/src/main/kotlin/mcp.publishing.gradle.kts

@@ -46,7 +46,7 @@ mavenPublishing {
 }
 
 private fun Project.configureSigning(mavenPublishing: MavenPublishBaseExtension) {
-    val gpgKeyName = "GPG_SIGNING_KEY"
+    val gpgKeyName = "GPG_SECRET_KEY"
     val gpgPassphraseName = "SIGNING_PASSPHRASE"
     val signingKey = providers.environmentVariable(gpgKeyName)
         .orElse(providers.gradleProperty(gpgKeyName))