[Nix] Cache PATH between nix develop invocations

Summary:
For certain environments such as XCode, it would be nice to be able
to pull in the correct version of tools needed for building native
dependencies.

Have nix write the PATH to a file, so we can source it later.

https://linear.app/comm/issue/ENG-1950

Test Plan:

# trigger `nix develop`
nix develop --command true

cat ~/.cache/comm/path
# should show `export PATH=...`

source ~/.cache/comm/path
protoc --version # should be 3.15.8

Reviewers: atul, varun, abosh, O1 Nix

Reviewed By: atul, O1 Nix

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D5319