Introduce a way to ignore the results of icount regression tests, by
specifying `allow-regressions` in the pull request body. This should
apply to both pull requests and the merges based on them, since `gh pr
view` automatically handles both.
`--limit=1` seems to apply before `jq` filtering, meaning our
`WORKFLOW_NAME` ("CI") workflow may not appear in the input to the jq
query. Removing `--limit` provides a default amount of inputs that jq
can then filter from, so this works better.
Add support in `ci-util.py` for finding the most recent baseline and
downloading it, which new tests can then be compared against.
Arbitrarily select nightly-2025-01-16 as the rustc version to pin to in
benchmarks.
