Fixes issue where PATH=$PATH gets escaped as PATH=\$PATH

The current version of fabric appends $PATH to the path variable before exporting it with commands to run.

For example:

[export PATH=\$PATH:\"~/api\"  && rsync ].    The '\' character before $PATH causes rsync command to fail since the path is not properly set.

This change fixes the method where the $PATH is appended after expansion.