Add links after building

This needs a Groovy Postbuild script:

	isBuilding = false
	build = manager.build
	build.getParentBuild().getRuns().each() {
		run ->
		if (!run.equals(build) && run.isBuilding()) {
			isBuilding = true
		}
	}
	projectRoot = manager.build.getParentBuild().getProject().getBuildDir()
	postBuild = new hudson.FilePath(projectRoot).child("../workspace/label/master/postbuild.groovy")
	if (!isBuilding && postBuild != null && postBuild.exists()) {
	  shell = new GroovyShell(new Binding(manager: manager))
	  shell.evaluate(new java.io.InputStreamReader(postBuild.read()))
	}

The reason is that we will only know *after* the build steps whether the
build has been successful. That's the job of the postbuild script.

We need to take care, however, to run this only when all the axes have
been built. Hence we skip the postbuild until all axes are done.

Also, since the postbuild script is run on the main Jenkins node, we need
to make sure to access the correct node's workspace to find the script
(unfortunately, MatrixRun#getWorkspace() does not return the workspace of
the node associated with that particular MatrixRun, hence we have to use
knowledge that the 'master' node actually runs on the same machine).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>