"su: illegal option -- s" on OS X with shell executor and --user

When integrating with an OS X runner using the shell executor and the --user option, the build fails:

gitlab-ci-multi-runner 0.5.0 (c38415a)
Using Shell executor...
su: illegal option -- s
usage: su [-] [-flm] [login [args]]

ERROR: Build failed with: exit status 1

I've found this in the debug log:

command: su
arguments:
- --shell
- /bin/bash
- --login
- gitlab-runner

Looks like the su command is run with the --shell and --login options which aren't supported in OS X (see man su).