Add support for virtualenv in installer

[Pixel-Minions]

Fixes #1641

Add support for virtualenv in install.py.

[linux-foundation-easycla[bot]]

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[Pixel-Minions]

Thanks @Pixel-Minions!

You will need to sign the CLA and sign-off your commit before we can merge your pul reuqest.

See #1643 (comment) for instruction on how to sign the CLA. See https://github.com/AcademySoftwareFoundation/rez/pull/1643/checks?check_run_id=21550026398 for how to sign-off your commit.

@JeanChristopheMorinPerso Also, I have signed the EasyCLA, but it seems I need approval or something like that. I have done it twice.

[Pixel-Minions]

@JeanChristopheMorinPerso Apologies, closing, got messy. I will reopen another.