FilePath is not persistable as a configuration.

... because it contains references to the transient objects like a channel connection to a slave. So at minimum the field needs to be marked 'transient'.

Builder can be also shared across multiple builds, so the best thing to do in this case is not to store this state in the field. There's an up-to-date check inside the performInstallation method so that this lack of caching shouldn't really result in a notcable performance degradation.
Relaxing the form validation a little bit.

If the variable references are used inside the setting, they will cause the validation to fail and issue a false positive warning, and that'd be confusing.
I'm fixing this in FilePath.validateFileMask, but in the mean time a workaround is implemented in a plugin.
Remove some comments that no longer apply.
Merge pull request #1 from jenkinsci/nitpicking

Nitpicking