[keyserver] Use [[ ]] instead of [ ] in source-nvm.sh

Summary:
Relevant Linear issue here. Replaced [ ] with [[ ]] in source-nvm.sh.

Depends on D4429

Test Plan: N/A

Reviewers: ashoat, atul

Reviewed By: ashoat, atul

Subscribers: palys-swm, Adrian

Differential Revision: https://phab.comm.dev/D4422