cargo build should track environment variables

[ju1ius]

Hi,

Currently when we want to switch the PHP version for a build, we have to run

cargo clean
PATH="/path/to/build:${PATH}" cargo build

Which wipes out the entire compilation cache.

The build script should use cargo:rerun-if-env-changed=PATH so that we can just do:

PATH="/path/to/build:${PATH}" cargo build

and cargo rebuilds only what's needed, not the entire dependency tree.