Add form validation warnings for parameter names matching known env vars

[daniel-beck]

I considered putting this directly in ParameterDescriptor, but I think that parameters don't have to create env vars matching their name, so making this opt in is pragmatic IMO.

Testing done

Manual only so far, PR build to see whether the Jelly changes work.

Proposed changelog entries

• human-readable text

Proposed upgrade guidelines

N/A

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