gitlab-ci.yml: Fix the filtering for the git submodules

Commit 7d7dbf9d introduced a new line starting with
"GIT_SUBMODULES_ACTION=" in the config-host.mak file. The grep that
tries to determine the submodules in the gitlab-ci.yml file matches
this new line, too, causing a warning message when updating the modules:

 warn: ignoring non-existent submodule GIT_SUBMODULES_ACTION=update

Fix it by matching the "GIT_SUBMODULES=..." line only.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Message-Id: <20210331073316.2965928-1-thuth@redhat.com>
Message-Id: <20210401102530.12030-11-alex.bennee@linaro.org>