Fix issue with shebang length being too long in pip executable

[tnguyen14]

I was trying to run docker-selenium test suite on my Jenkins instance, and ran into an issue where the pip executable's shebang length being too long. Jenkins tends to have very long directory path. Here's an example:

#!/bb/jenkins/workspace/icense_docker-selenium_PR-1-GNCWWURZ32MIKRWRAGOJLHHRNQDNWKSNBSNTKV5GAN44JPAJLVLQ/tests/docker-selenium-tests/bin/python2

The error:

-bash: /bb/jenkins/workspace/icense_docker-selenium_PR-1-GNCWWURZ32MIKRWRAGOJLHHRNQDNWKSNBSNTKV5GAN44JPAJLVLQ/tests/docker-selenium-tests/bin/pip: /bb/jenkins/workspace/icense_docker-selenium_PR-1-GNCWWURZ32MIKRWRAGOJLHHRNQDN: bad interpreter: No such file or directory

This fix uses the suggestion suggested in pypa/virtualenv#596 (comment)

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/cc @diemol

[diemol]

Thanks for your contribution @tnguyen14!