[CHANGE] Share of results should only happen when running GitHub Actions in the `main` branch

[etagwerker]

#203 moved the benchmarks from Travis CI to GitHub Actions CI. It also started sharing results to https://ips.fastruby.io (which is an instance of benchmark.fyi) for every commit to a PR and every push to main.

For example:

Source: https://ips.fastruby.io/R4

It would be ideal if sharing of results only happened when we committed and pushed changes to main. That would avoid some unnecessary records on https://ips.fastruby.io

It should be a pretty straightforward tweak to the GitHub Actions CI configuration (?) which is here:

https://github.com/fastruby/fast-ruby/blob/main/.github/workflows/benchmarks.yml#L3-L13