fix outdir for nim PR

[timotheecour]

/cc @jangko
seems like this is the only nimble package that fails for this nim PR: nim-lang/Nim#13382
after this fix, nim-lang/Nim#13382 should have its CI pass

let me know if you have any questions

[jangko]

it does not pass the CI test, somehow it didn't trigger CI build.

[jangko]

the $projectdir in --outdir:'$projectdir' not replaced by actual path on Windows

[timotheecour]

/cc @jangko hmm, that's too bad... I'm seeing a few possible choices to fix this:

• add a tests/test_json.cfg file containing:
  --outdir:"$projectdir"
  not sure whether this will help, depends on whether the windows fail was due to quoting issues or to $projectdir substitution just not working at all on windows
• expose getNimbleFile via a magic in nim (I added it for internal use for docgen in fix #13150 nim doc --project now works reliably nim-lang/Nim#13223 but it would actually be useful to solve this, giving a reference path that works however the package is installed (locally via nimble develop or installed via nimble install0
• change a bit the logic in test_json.nim

thoughts?

EDIT: see #47