Show PATH changes made by R installation logic

Not seeing them makes diagnosing issues like https://travis-ci.community/t/in-r-version-4-0-0-library-path-not-writable/9744 and https://travis-ci.community/t/r-support-failed-on-xenial/9822 impossible.

Ivan Pozdeev authored and committed

Nothing here #76.2 passed

  • Ran for
AMD64
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