Configure pip to disable pulling in dependencies

[jakirkham]

Fixes #2748

We never want pip to install dependencies in the build as we will supply them ourselves.

[jjhelmus]

Does it make sense to add env["PIP_NO_BUILD_ISOLATION"] = False as well to disable build isolation?

Nevermind, just saw that it is already there.

[jakirkham]

Yeah was just trying to figure out if we were doing that or not earlier as well. Seems like we are, which is good.

[jakirkham]

Looks like there are a couple CI failures related to some rpm test where the HTTP request fails. Seems unrelated. Also one Travis CI test timed out.

[msarahan]

Added some small comments about the counterintuitive logic of the "NO" env vars. Thanks @jakirkham

[jakirkham]

Thanks @msarahan.

Yeah the "NO" environment variables are still very weird. In addition to the counterintuitive logic, some of them will crash if set incorrectly (as they expect more arguments).

xref: pypa/pip#2897

[msarahan]

Frustrating, but at least they are consistent.

[github-actions]

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