Allow users to set their own env vars per run-configuration.
It is not respected everywhere (see the list below), but it works on our example projects.

Main features:
- Env vars should work when **run**ing and **debug**ging any binary.
- Env vars should work when **run**ning any test.
- Env vars should work when **debug**ging Go and Java tests. Env vars for C++ test debugging are implemented internally, but they're part of a larger patch and I'll need some more work to untangle that.