Auto merge of #85362 - jsgf:fix-emit-metadata, r=estebank

Use command line metadata path if provided

If the command-line has `--emit metadata=some/path/libfoo.rmeta` then
use that.

Closes #85356

I couldn't find any existing tests for the `--emit TYPE=PATH` command line syntax, so I wasn't sure how to test this aside from ad-hoc manual testing. Is there a ui test type for "generated output file with expected name"?