Summary:
This fixes an issue introduced in 0c56fc4d whereby the location of Python is evaluated many times and leads to excessive logging of unknown python locations of CentOS 6.

The location is now only checked once.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7123

Reviewed By: zhichao-cao

Differential Revision: D22532274

Pulled By: ajkr

fbshipit-source-id: cade71b4b46e9a23d63ecb4dd36a4ac8ae217970