add missing config for Linux-environments

[4ch1m]

This is a bug for Linux-systems, which will crash the script when a pre-/post-build script is being used.

The (dictionary-)entries env and path never get set/initialized in KickAssembler(C64).sublime-build when running on a Linux-environment.
And then the script crashes when it tries to access env.update().

    def addPrePostVarsToDict(self, sourceDict, buildMode):
        prePostEnvVars = {
            "kickass_buildmode": buildMode,
...
            }
        sourceDict.get('env').update(prePostEnvVars)
        return sourceDict

This PR will fix this by simpy initializing these dict-entries with empty values.

[Swoffa]

Suggested change

	"path": ""
	"path": "$PATH"

For safety, if PATH would be initialized.

[Swoffa]

Thanks!
See comment, then we can merge. :)

[4ch1m]

Yep, that's definitely better. 😃
Fixed.
Thanks, Swoffa.