Make Stack's integration tests not depend on $PATH

While looking for a test covering problem resolved in #4467 I have found that our integration tests don't clean $PATH variable and thus could get run in an environment with some GHC being availble on user's path. It would be better if they were run in an "empty" environment probably with just Stack itself being available.
In my 1st attempt to do that I discovered that `basic-install` test appears to fail on my machine when `$PATH` environment variable gets deleted.
We need to find why this happens and check and fix problems with other tests.