In future commits, we'll need to modify the commands used depending on
the platform (and some other factors as well). Storing them in a list
will make this easier to manipulate.

This will facilitate Windows support, which will be added in a
future commit.

This is needed to support pyenv on Windows.

This will be needed to fully support Windows, which will happen in a
future commit.

This is needed to support pyenv on Windows, which will be added in a
future commit.

This will be needed to support pyenv on Windows, which will happen in a
subsequent commit.

To support Windows (which will be done in a subsequent commit), we'll
need to translate Windows-style paths to Cygwin-style paths (for
example, C:\foo\bar -> /c/foo/bar). The fileExists step only supports
Windows-style paths when run on Windows, which means that we need to
actually run pyenv to assert that it exists.