.gitlab-ci.yml: avoid using != to compare variables to support old versions

`!=` was only introduced after 11.11, according to doc at:

https://docs.gitlab.com/ee/ci/variables/README.html#environment-variables-expressions

For private gitlab instance, the version may not be new enough.
Use `==` to make it backward compatible.

Signed-off-by: Joe Guo <joeg@catalyst.net.nz>
Reviewed-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>

Autobuild-User(master): Ralph Böhme <slow@samba.org>
Autobuild-Date(master): Fri May 31 09:48:37 UTC 2019 on sn-devel-184
