packaging: don't use plain 'python' if another python has been specified
Before this change, packaging on CentOS 8 failed because 'python'
is used instead of 'python3'.
Change was tested with:
- make docker-centos7
- make docker-centos8
- make docker-ubuntu-bionic
Differential Revision: https://phab.mercurial-scm.org/D9464
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