PSA: emsdk will now attempt to clean up (environment variables) after itself

[Sam Clegg]

As of https://github.com/emscripten-core/emsdk/pull/801, emsdk will attempt to clean up stale environment variables that it thinks might be associated with inactive tools.    Hopefully this will strictly improve things but if you notice any issues please reply here or comment on the PR.

cheers,

sam