Support listing the pip/requirements.txt friendly output of dependencies for an env

[brettlangdon]

Something like:

$ riot freeze <hash>
sphinx~=4.5.0
sphinx-click~=3.1.0
sphinx-rtd-theme~=1.0.0
reno

The goal being you could do riot freeze <hash> > requirements.txt.

[P403n1x87]

What if we let riot shell take a -c option with a string to execute, so that we could do

riot shell -c "pip freeze" <hash> > requirements.txt

?