`--nim:path` not reliable: doesn't work with `exec "nim c ...`
* at very least (and in meantime) a caveat should be mentioned in `--help` that `nimble test --nim:path` is not 100% reliable, and explain that tests (or other tasks) that rely on `exec` (eg `exec "nim c ...`) will not be affected by `--nim:path` flag
* next, we can brainstorm ways to make `--nim:path` affect `exec "nim c ...`, possibly by another flag if needed (or same flag for simplicity)

EDIT: see proposal here: https://github.com/nim-lang/RFCs/issues/273
