Record build scan url directly from init script

Instead of writing the URL to a file on disk, reading it later and
using the Actions API to record the output parameter and write the notice,
these things are now done directly via Actions commands emitted directly
from the init script.

src/cache-gradle-user-home.ts

@@ -299,6 +299,9 @@ def registerCallbacks(buildScanExtension, rootProjectName) {
         def scanFile = new File("gradle-build-scan.txt")
         buildScanPublished { buildScan ->
             scanFile.text = buildScan.buildScanUri
+
+            println('::notice title=build-scan-url::' + buildScan.buildScanUri)
+            println('::set-output name=build-scan-url::' + buildScan.buildScanUri)
         }
     }
 }