Inject correct environment variables when they are discarded

[karkhaz]

At every process invocation during the build process, we should check if the environment variables are being preserved. If something important (like $PATH) has been clobbered, we should log a warning and re-introduce that variable.

This is so that we can get further in the build process, while still gaining insight into what errors occurred without our intervention. It is also so that, in the paper, we can present a prototype mitigation and claim that the mitigation results in x% more builds succeeding.