Publish a build scan only if authenticated

Previously, anyone could publish a scan and publishing was performed
if the GRADLE_ENTERPRISE_URL environment variable was set.

ge.spring.io has now been locked down to prohibit anonymous build scan
publishing. This commit aligns with this change by only attempting to
publish a build scan when authenticated with the server.

Authentication is achieved via an access token that is made available
via an environment variable on CI and locally via a file in ~/.gradle.

One can obtain an access key by running the following command:

./gradlew provisionGradleEnterpriseAccessKey

Closes gh-24371

build.gradle

@@ -17,7 +17,8 @@ buildScan {
 	obfuscation {
 		ipAddresses { addresses -> addresses.collect { address -> '0.0.0.0'} }
 	}
-	publishAlwaysIf(System.getenv('GRADLE_ENTERPRISE_URL') != null)
+	publishAlways()
+	publishIfAuthenticated()
 	server = 'https://ge.spring.io'
 }