sysutils/google-compute-engine-oslogin: fix OSLogin

Larry Rosenman and Glen Barber from the FreeBSD team reported that the OSLogin
feature has not been working since the last update I made to
sysutils/google-compute-engine-oslogin ports package. I found out that the
problem is in some modifications made by upstream in the shell script
responsible to activate and deactivate the feature. I prepared the attached
patch to fix it. With the patch applied in a FreeBSD 13 instance on GCE I was
able to use the OSLogin feature without any problem. I also sent this same
patch upstream.

PR: 238561
Submitted by: lucas.kanashiro@collabora.com (maintainer)