dvc: remove PATH prefixes appended by pyenv

[efiop]

Unlike PyInstaller, pyenv doesn't leave backups of original env vars
when it modifies them. If we look into the shim, pyenv and pyenv-exec,
we can figure out that the PATH is modified like this:

PATH=$PYENV_BIN_PATH:${bin_path}:${plugin_bin}:$PATH

where

PYENV_BIN_PATH - starts with $PYENV_ROOT, see pyenv-exec source code.
bin_path - might not start with $PYENV_ROOT as it runs realpath on
    it, see pyenv source code.
plugin_bin - might contain more than 1 entry, which start with
    $PYENV_ROOT, see pyenv source code.

So having this, we can make a rightful assumption about what parts of the PATH
we need to remove in order to get the original PATH.

Fixes #2506

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