Misuse of local java.io.File

`GrailsInstallation.getExecutable` does not make sense in the context of distributed (slave) builds. `ToolInstallation.getHome` is a path on the slave node, so constructing a `java.io.File` from it in code run on the master is in general senseless—especially if master is Windows and the slave is Unix, or vice versa. Taking that `File.path` and using it on another machine might happen to work by accident but it cannot be guaranteed.

`getExists` is similarly meaningless, though it appears to be unused anyway.

Change tested successfully with Linux master and XP slave.