Install blocked by `noexec` set on `/tmp` directory

The `install` hook fails on clouds with CIS Hardening implemented due to the `/tmp` directory mount option "noexec".

When installing "fabric" via pip, the dependency "pynacl" fails during compilation. [0] This occurs because it is attempting to load a symbol from a shared object in the `/tmp` directory. This is a similar problem to bug 1935032.

The following modification to the command in the `install` hook works:
. /opt/juju_venvs/duplicity/bin/activate; TMPDIR=/var/lib/pip https_proxy=XXX pip install fabric

[0]: https://pastebin.canonical.com/p/nCXnqdCVSx/