Check production install only if async is needed

[doruchan]

Hello,

This is spawn from #1500.

In the PackageCache.add_variants method there's a check for verify that rez is installed as a "production install".
I reckon this check only needs to happen if we want to cache the packages in async mode, and it should not matter if we are caching them in the same thread.

Thank you!

[linux-foundation-easycla]

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[manuelkoester]

👍 We have a similar fix on our end, but including the same check additionally at https://github.com/AcademySoftwareFoundation/rez/blob/3.4.0/src/rez/resolved_context.py#L1938

[maxnbk]

I've reviewed this pair of PRs (this one, and the one @manuelkoester produced in #2151 ), and judged manuel's to be a strict superset that covers more code, which turns out to be a good idea because of the resolved guard in resolved_context.py.

I'm therefore going to close this PR in favor of @manuelkoester 's, and move review over to that thread. Thank you for the submission anyway, @doruchan , there's not per-se anything wrong with it, but we would likely have asked for tests anyway and manuel's has them, so it saves a back-and-forth loop.

[doruchan]

Thank you Max, no worries I am glad you are merging @manuelkoester! Thank you!