Split the benchmark workflow

[michaelpj]

The pre and post jobs don't need the benchmarking machine, and the
benchmarking machine bit that runs code from the repo has reduced permissions.

The logs are also available as an artifact.

Pre-submit checklist:

• Branch
  • Tests are provided (if possible)
  • Commit sequence broadly makes sense
  • Key commits have useful messages
  • Changelog fragments have been written (if appropriate)
  • Relevant tickets are mentioned in commit messages
  • Formatting, PNG optimization, etc. are updated
• PR
  • (For external contributions) Corresponding issue exists and is linked in the description
  • Targeting master unless this is a cherry-pick backport
  • Self-reviewed the diff
  • Useful pull request description
  • Reviewer requested

[michaelpj]

/benchmark plutus-benchmark:nofib

[github-actions]

Click here to check the status of your benchmark.

[kwxm]

I think this is obsolete now.

[michaelpj]

I do actually still think this is a good idea but it's not a big deal