CI: Fix merge_group GPU benchmark caching on gh-pages branch

[samuelburnham]

Problem

Currently we write the comparative benchmark output of a merged PR to gh-pages, in order to reuse it as the baseline for the next PR. However, these benchmark JSONs can't be easily published to our gh-pages site due to incompatible BenchmarkId formats. The gh-pages benches need a date and SHA as params to be formatted correctly, while the commit-comment benches run on merge_group need the SHA in the name for differential comparison.

Solution

We need to either reconcile the two formats and publish the commit-comment benches to gh-pages (ideal solution) or move the cache somewhere else like https://github.com/actions/cache.