Output buildscan URL from CI to Slack

[philwebb]

It would be nice if we could record the Gradle buildscan URL and include it in the Slack message.

[wilkinsona]

We can write the scan URL to a file with something like this:

gradleEnterprise {
    buildScan {
        buildScanPublished { PublishedBuildScan scan ->
            file("build-scan-uri.txt") << "${scan.buildScanUri}\n"
        }
    }
}

Hopefully we can then read the URL from the file and include it in the Slack alert.