getdeps: fixup python3 usage on GitHub Actions CI
getdeps: fixup python3 usage on GitHub Actions CI

Summary:
My recent change to ensure that we were using python3 to launch everything failed on windows: the GH actions environment has `python.exe` in the path and it is python version 3.  There is no such thing as `python3` in that environment, although there is a `python2`.

Refs: https://github.com/facebook/watchman/pull/797

Reviewed By: chadaustin

Differential Revision: D20740411

fbshipit-source-id: 0e40590ccedc18e327ebb84901e2509588fdd0ff

ReactiveSocket/reactivesocket-cpp,ReactiveSocket/reactivesocket-cpp,rsocket/rsocket-cpp,rsocket/rsocket-cpp,ReactiveSocket/reactivesocket-cpp,rsocket/rsocket-cpp,rsocket/rsocket-cpp