[Nix] Set TMP to stable path

Summary:
mysql2 (or some dependent) node package assumes that the temporary
directory will have a stable path prefix, so we'll provide it with one.

https://linear.app/comm/issue/ENG-2366

Test Plan:

nix develop

cd keyserver
yarn dev

# Not actually sure how to trigger previous failure

Reviewers: adar, varun, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D5745