Fix install-as-rez-package script for Windows

[davidlatwe]

Problem

1. Python executable path was assembled by os.path.join(tmpdir, "bin", "python"), which will not work on Windows. (dir name is scripts instead of bin).

2. repo_path that has \\ in it will not be properly escaped.

Solution

1. Replace hardcoded bin dir path with virtual env helper function.
2. Format repo_path with repr()