xtask: Add OVMF_CODE/OVMF_VARS env vars

[nicholasbishop]

The search order for finding the OVMF files is now:

1. Command-line argument (--ovmf-code, --ovmf-vars)
2. Environment variable (OVMF_CODE, OVMF_VARS)
3. OS-dependent paths

Checklist

• Sensible git history (for example, squash "typo" or "fix" commits). See the Rewriting History guide for help.
• Update the changelog (if necessary)

[nicholasbishop]

@phip1611 I wanted to suggest changing slightly how the env vars in #690 get checked so that it happens after checking the CLI args, but figuring out how to type it up in a comment was harder than just putting up a PR, so here it is :)

[phip1611]

Nice, LGTM