Fixing up some format strings in nox configs.
Fixing up some format strings in nox configs.

Using `STRING_TEMPLATE % VARIABLE` can introduce hard-to-find bugs
if `VARIABLE` is expected to be a string but ends up being a tuple.
Instead of using percent formatting, just using `.format`.

Also making tweaks to `get_target_packages` to make some path
manipulation / checks OS-independent.

googleapis/python-monitoring,googleapis/python-monitoring