Introducing new actions for the GitHub dependency graph will involve reuse of much of
the action infrastructure. This commit reorganises things a little to facilitate reuse.
Although convenient, the os.homedir() function can return a different value
that the 'user.home' SystemProperty in Java. The latter is used to locate
the Gradle User Home directory.
By switching to use Java to determine the value for 'user.home', we can use
the same process as Gradle to determine Gradle User Home.
Fixes#207
