CI: broken merge_group tests

[huitseeker]

Issue

Merge group GPU benchmark tests are broken:
https://github.com/lurk-lab/lurk-rs/actions/runs/6777848351/job/18422227933

Run cat fc495987b972a3736da3d946e5615590c06036.json | criterion-table > BENCHMARKS.md
  cat fc495987b972a3736da3d9146e56155920c06036.json | criterion-table > BENCHMARKS.md
  shell: /usr/bin/bash -e {0}
  env:
    CACHE_ON_FAILURE: false
    CARGO_INCREMENTAL: 0
    BASE_REF: 25c170bf85cd0676fd012[3](https://github.com/lurk-lab/lurk-rs/actions/runs/6777848351/job/18422227933#step:17:3)7acdf[4](https://github.com/lurk-lab/lurk-rs/actions/runs/6777848351/job/18422227933#step:17:4)0e1632d39[5](https://github.com/lurk-lab/lurk-rs/actions/runs/6777848351/job/18422227933#step:17:5)9[8](https://github.com/lurk-lab/lurk-rs/actions/runs/6777848351/job/18422227933#step:17:8)
    LURK_BENCH_OUTPUT: commit-comment
An error occurred processing Criterion data: Duplicate column: fib-branch=gh-readonly-queue

Which is logically followed by Error: Missing comment 'body' or 'body-path'. on the next step (commit-comment)

Origin

Introduced in #852 and made visible by #857.

[huitseeker]

I've added a gist of :

LURK_RC="100,600" LURK_BENCH_OUTPUT=commit-comment cargo criterion --bench fibonacci_lem --message-format=json | tee -a $(git rev-parse HEAD).json

On a machine that does not have enough RAM to run past rc=100. At:
https://gist.github.com/huitseeker/2b2ffc93d2963dda8e1d920726b375f8

Note the tee -a is load-bearing to reproduce this, which I believe is caused by nested checkouts: in the case of our gh-merge queue branch, the branch is likely the same both on base ref and PR.

Passing it to criterion-table does produce

An error occurred processing Criterion data: Duplicate column: fib-branch=master

Seemingly, using the bench group as a separator with the function name duplicated is not helpful when using a single json, contrarily to two (differential bench), which is what I used to demo the output in #852.