Lock the cache

[DanielKeep]

Given that rust-lang/cargo#354 is now fixed, and I complained about Cargo not locking its files, it seems only fair that cargo-script should properly lock its own cache.

One complication is the shiny new shared output folder support; that will probably need to be locked independently of everything else. Should check to see what Cargo does here, first, but we need to be careful due to the existence of .meta-hash files.