[Nix] Fix whitespace quoting for path

Summary:
If a user has a path with whitespace, avoid having the
string split when passed to export command.

Test Plan:

export PATH="/a b:$PATH"

nix develop

source ~/.cache/comm/path
# don't get error from export

Reviewers: atul, varun

Reviewed By: atul

Subscribers: ashoat, tomek, abosh

Differential Revision: https://phab.comm.dev/D5488