chore: tidy up tz-solve-osemosys-build-webhook workflow

[ollie-bell]

Description

Remove unwanted verbosity from curl command in trigger webhook workflow + extra tidy up.

Also testing workflow will trigger on PR merge.

Checklist

• Dependencies install correctly in a clean environment and code executes;
• Test coverage extended; created tests fail without the change (if possible);
• All tests passing;
• Commits follow a type convention (e.g. https://gist.github.com/brianclements/841ea7bffdb01346392c#type);
• Extended the README, documentation and/or docstrings, if necessary;