bencher: Clone thresholds from main (#38220)

When running bencher on pull requests or try, we want to clone any
thresholds we setup for main to detect regressions.
Note: The thresholds we are actually interested in are defined on the
production profile, while in PRs we currently test release. Probably we
should reconsider that, but perhaps that could be a different PR with
seperate discussion.

Testing: ./mach try
([ohos](https://github.com/servo/servo/actions/runs/16779272577))

---------

Signed-off-by: Jonathan Schwender <schwenderjonathan@gmail.com>
Signed-off-by: Jonathan Schwender <55576758+jschwe@users.noreply.github.com>
Co-authored-by: sagudev <16504129+sagudev@users.noreply.github.com>

.github/workflows/bencher.yml

@@ -116,6 +116,7 @@ jobs:
             --branch-start-point ${{ github.base_ref }} \
             --branch-start-point-hash ${{ github.event.pull_request.base.sha }} \
             --branch-reset \
+            --start-point-clone-thresholds \
             --github-actions ${{ secrets.GITHUB_TOKEN }}" >> "$GITHUB_ENV"
       - name: Set bencher opts for main
         if: ${{ github.event_name == 'push' && github.ref_name == 'main' }}
@@ -132,6 +133,7 @@ jobs:
             --hash $(git rev-parse HEAD~1) \
             --branch-start-point main \
             --branch-start-point-hash $(git merge-base upstream/main HEAD) \
+            --start-point-clone-thresholds \
             --branch-reset" >> "$GITHUB_ENV"
       # we join results and send all data once to have it all in one report
       - name: Send results