Make Stack's integration tests not depend on $PATH

[qrilka]

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.

[mihaimaruseac]

Thank you for discovering this

[dbaynard]

Test #4268 is related (see our discussion on #4407). With a clean path the test becomes trivial.

[mihaimaruseac]

I think we can adapt the test to setup PATH properly before running (needs commenting out and issue created and then PR)

[dbaynard]

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.

@qrilka Where is this branch? I'd like to see how you went about these changes. Thanks!

[qrilka]

I think it was a (now deleted) branch for #4467 but if I remember it correctly you don't need a specific branch to see that effect

[dbaynard]

Ah, thanks anyway.