[Services] Use GNU + BSD compatible find command

Summary:
Use -type d to make the command not error with GNU findutils.

Part of https://linear.app/comm/issue/ENG-2651

Test Plan:

On a mac, which has BSD Tools
$ cd services
$ ./scripts/list_services.sh

$ nix develop
$ ./scripts/list_services.sh

# Should produce the same output

# Run calling script
yarn run-integration-tests blob

Reviewers: varun, bartek, max

Reviewed By: bartek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6192