Fix bug in CLI arg/env var prioritization + corresponding tests

[BenMusch]

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Fixes bug in #5738
Fixes #5736

Explanation of what happened:

• When I changed the names of the :env_var value to :env, I left one unchanged
• The tests were passingly misleadingly because the stubbing order made it so that all env vars were actually ignored (and one of my tests was testing the non-env-var behavior due to my mistake in the test)

[scpeters]

I confirmed that the updated test fails without the corresponding fix.

[MikeMcQuaid]

Thanks again @BenMusch!