gitlab/ci_template: Fix meson args
When a variables:
block is defined, it overwrites all the other
ones, including the global one, whichs causes the variables set
there to be empty. The reason it works with some variables
is that they seem to be Runner-setup variables, that are
preprocessed by the runner, ex GIT_STRATEGY, and it doesn't matter
if they end up in the job's environment.
Quoting from the docs [1]
When the variables keyword is used on a job level, it overrides the global YAML variables and predefined ones.
[1] https://docs.gitlab.com/ce/ci/yaml/README.html#variables