Add CI variable RUNNER_TAG

This allows to set a job tag dinamically.
We need this to be able to select the Kubernetes runner.
See https://wiki.qemu.org/Testing/CI/KubernetesRunners.

Signed-off-by: Camilla Conte <cconte@redhat.com>
Message-Id: <20230522174153.46801-5-cconte@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

.gitlab-ci.d/qemu-project.yml

@@ -1,6 +1,13 @@
 # This file contains the set of jobs run by the QEMU project:
 # https://gitlab.com/qemu-project/qemu/-/pipelines
 
+variables:
+  RUNNER_TAG: ""
+
+default:
+  tags:
+    - $RUNNER_TAG
+
 include:
   - local: '/.gitlab-ci.d/base.yml'
   - local: '/.gitlab-ci.d/stages.yml'