Xcode not finding my GitLab ssh key all of a sudden

Yesterday, Xcode could resolve a private Swift Package Manager (SPM) dependency hosted on our GitLab account. Today it can’t, complaining that “Authentication failed because the credentials were missing”.

I’ve never given Xcode my GitLab credentials or a PAT. I do have my GitLab private key added to the ssh agent on macOS, which ssh-add -l confirms. Nothing has changed there, and this has worked across a few reboots since I set this machine up a month and a bit ago. This is the relevant part of my ~/.ssh/config:

Host gitlab.com
        AddKeysToAgent yes
        UseKeychain yes
        IdentityFile ~/.ssh/id_onbeams

This should be enough, yes?

The only thing that’s changed is I upgraded to macOS Big Sur 11.2.1 (from 11.2) this morning. Has anyone else noticed this has broken their SSH Agent and/or Xcode authenticating via it?

A college suggests this might be the issue I’m hitting and it sure seems like it, but the solution in that thread (nuking IDESourceControlKnownSSHHostsDefaultsKey from ~/Library/Preferences/com.apple.dt.Xcode.plist and rebooting) doesn’t fix it for me.