Patches on the asv workflows

[camposandro]

Applies two small fixes on the asv-main and asv-nightly workflows:

• The asv-nightly workflow is failing because we're trying to compare two commit hashes and their benchmark results are not present in the action's working directory. Previously this was not a requirement but for asv v0.0.6+ we're seeing exceptions about it (example).

• According to the asv 0.6.1 changelog both asv run and asv continuous return a non-zero exit code if any of the benchmarks fail. In the case of asv-main, we still want to allow the workflow to finish so that the results are stored and the dashboard is updated (hence || true).

It closes #62, issue created for hipscat but that will in fact affect all projects based on the template.

Checklist

• This PR is meant for the lincc-frameworks/python-project-template repo and not a downstream one instead.
• This change is linked to an open issue
• This change includes integration testing, or is small enough to be covered by existing tests