Build script should be rerun when input environment variables are changed

[Osspial]

We used to do this, but that behavior was disabled in 942d853 because it doesn't play well with setting environment variables in cargo make and trying to use rust-analyzer at the same time. cargo make would set the environment variables and do a successful build, but when rust-analyzer would run cargo check it would force cef-sys and all dependencies to be rebuilt since it didn't have the environment variables set.

Relevant issues:
rust-lang/cargo#6834
rust-lang/cargo#4001