Support generating multiple file keys at once

[vyasr]

We should pass nargs parameter to make the --file-key CLI arg support generating a file with multiple keys. That will make it much easier for us to create complete environments for cases like unisolated wheel builds where we need multiple sets of dependencies installed into the same environment.

[jameslamb]

I'm going to try this. (ref: rapidsai/cuspatial#1473 (comment))

[GPUtester]

🎉 This issue has been resolved in version 1.16.0 🎉

The release is available on:

• GitHub release
• PyPI release
• Conda release - rapidsai
• Conda release - rapidsai-nightly

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