Make the "Copy link" button fixed width to avoid jank
Created by: chrismwendt
It seems some tokens shift in popover width when copying the link, probably due to the difference in the “Link copied” and “copied” length. But some don’t shift.
From Slack.
See https://github.com/sourcegraph/sourcegraph/pull/36039
Test plan
Manual
App preview:
Check out the client app preview documentation to learn more.