[Nix] Fix undeclared failure

Summary:
'set -u' will error if a variable is unset. Add bash
substitution to prevent the undefined variable causing an error.

https://linear.app/comm/issue/ENG-2838

Test Plan:

PS1= ./scripts/prompt_macos_direnv.sh

Reviewers: varun, atul

Reviewed By: varun, atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D6404