use env_clear

[wolfv]

cc @DerThorsten maybe that would help with the issues you had regarding deactivation. This should start a more isolated shell for building & testing.

One can still inject environment variables "from the outside" by using the env part of the script object, and the ${{ env.get(...) }} to grab them from the current process.

[baszalmstra]

Nice! This makes sure things dont accidentally leak into the build environment!