SConstruct : Locate `oslc` explicitly.
SConstruct : Locate `oslc` explicitly.

- On Windows, subprocess can't modify `PATH` using `env`
- See Warning about locating the executable. https://docs.python.org/3/library/subprocess.html#popen-constructor
- Don't use `which` on Linux because it is not supported in Python 2. The conditional can be removed when Python 2 support is dropped.

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