Drop duplicate isSnapshot variable in Gradle build script (#3413)

Publishing conventions plugin calculates `isSnapshot`:

    isSnapshot = project.version.toString().contains("SNAPSHOT")

twice:

  1. at the top of build script junitbuild.publishing-conventions.gradle.kts
  2. in configuration of `Sign` tasks.

Drop the second `val isSnapshot` to make `Sign` task configuration
re-use the variable from the top of the script.

gradle/plugins/common/src/main/kotlin/junitbuild.publishing-conventions.gradle.kts

@@ -47,7 +47,6 @@ signing {
 }
 
 tasks.withType<Sign>().configureEach {
-	val isSnapshot = project.version.toString().contains("SNAPSHOT")
 	onlyIf {
 		!isSnapshot // Gradle Module Metadata currently does not support signing snapshots
 	}