ci: build.sh: Drop the CI prefix from the CI_{MESON,NINJA}_ARGS vars

ci: build.sh: Drop the CI prefix from the CI_{MESON,NINJA}_ARGS vars

Although it is currently consistent with the other variables we define
when running ci in a local container environment, it isn't consistent
with the variable naming we use in GitLab recipes. Since the idea is
to unite the two, we're likely going to drop a few other variables from
the local env configuration anyway, hence this renaming.

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>