CI build displayed time is wrong with force push

# Description

When force pushing onto a merge proposal, the CI correctly runs again, but the displayed time of the last build is confusing.

You can see on the screenshot attached that the last build shows 17 hours ago, but it actually just run a few minutes ago when I forced pushed on my own branch to update the merge proposal, and that is confirmed when clicking on the build link.

# Steps

1) open a merge proposal on a repo with CI enabled and running
2) force push on your fork's branch
3) a new CI is triggered, but the displayed time is wrong

Merge proposal: https://code.launchpad.net/~hyask/meta-release/+git/meta-release/+merge/455210
Real last build: https://code.launchpad.net/~hyask/meta-release/+git/meta-release/+build/63439