[BUG] $pullRequestNo not interpolated in signed-commit-message

[pushcx]

Describe the bug
The DCO bot does not interpolate the PR number as described in the docs.

To Reproduce
I set signed-commit-message to "DCO 1.1 certified by $contributorName in PR $pullRequestNo" here. (I am going to update this to include $owner and $repo after filing this, so this is a permalink to what was there when the bug occurred.)

I made the attestation.

The bot committed the message: DCO 1.1 certified by pushcx in PR $pullRequestNo.

Expected behavior
I expected the message DCO 1.1 certified by pushcx in PR 1.

Diagnosis?
It looks like this interpolation was commented out. I don't understand why that would be deliberately done, so I'm opening a bug instead of a PR.