fix: minor issues with go_mod_path construction and use

[amoeba]

What's Changed

Fixes two minor things with go_mod_path. Without this, the generated dev.yaml will use a hard coded go/go.mod path. I missed this in aa2b3ac. The other thing is just a minor safety thing, that go_mod_path didn't have a default.

Tested locally with a test repo:

$ cat .github/workflows/generate.toml | grep subdir
subdir = "."
$ adbc-gen-workflow generate .
$ grep "go\." ".github/workflows/dev.yaml"
          cache-dependency-path: ./go.sum
          go-version-file: ./go.mod

[lidavidm]

At some point I need to handle #29, though I'm not quite sure what a good test even looks like (e.g. one that could even capture this)

[lidavidm]

Note that pre-commit is failing

[amoeba]

Pre-commit is fixed. Re-requested review since I can't merge.