Clean up quoting in windows build script

I noticed during separate work in issue https://github.com/whimboo/mozmill-environment/issues/51, the general quoting style in build.py for windows was a bit inconsistent. This pull request makes the file consistent, hopefully in the desired way based on my earlier approved work. Single quotes for internal strings, double quotes for customer-facing strings, and some other minor new line adjustments.

Adding @whimboo and @davehunt for visiblity.