creating a PR in a repo with `master` not `main` is still failing

An assumption that the default branch is `main` is baked in somewhere

https://github.com/fsprojects/FSharpx.Collections/issues/228

```
Base branch: main
Fetching base branch: main
/usr/bin/git fetch origin main
fatal: couldn't find remote ref main
Error: ✗ Message 12 (create_pull_request) failed: The process '/usr/bin/git' failed with exit code 128
```
