ci/piglit: Use structured tagging for Piglit

Structured tagging (cf. mesa/mesa!33421) captures a checksum of the
thing we think we're building, and verifies this through the chain.

When we run container builds, we check that the tag we've captured in
the CI variables matches the calculated checksum, to make sure the
declared tags are consistent and we always have traceability.

When we run tests, we check the tags again between what was declared in
the CI variables and what we're actually running from the test
container. This makes sure that we're always testing what we think we're
testing.

As a side advantage, the rule inheritance we need to make this work
means that we can start doing more optional downloads via overlays,
instead of pulling a whole container full of stuff we might not ever
use.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/34539>

.gitlab-ci/test/gitlab-ci.yml

@@ -181,6 +181,10 @@ yaml-toml-shell-py-test:
   variables:
     ANGLE_TAG: ${CONDITIONAL_BUILD_ANGLE_TAG}
 
+.test-piglit:
+  variables:
+    PIGLIT_TAG: ${CONDITIONAL_BUILD_PIGLIT_TAG}
+
 .b2c-vkd3d-proton-test:
   variables:
     HWCI_TEST_SCRIPT: install/vkd3d-runner.sh
@@ -195,6 +199,8 @@ yaml-toml-shell-py-test:
       - results/
     exclude:
       - results/*.shader_cache
+  extends:
+    - .test-piglit
   variables:
     # until we overcome Infrastructure issues, give traces extra 5 min before timeout
     DEVICE_HANGING_TIMEOUT_SEC: 600