ci/crocus: Mark unvanquished as flaky.

It's started rendering something different again, with a similar sort of
bad rendering to what's linked in the bug report (though this time it's a
'P' that became a white square).  Commit range 65b62db0..964323fe has
nothing particularly likely in it, so I expect this is some sort of cache
flushing fail or something.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21565>

src/gallium/drivers/crocus/ci/traces-crocus.yml

@@ -174,7 +174,9 @@ traces:
     crocus-g41:
       label: [unsupported]
     crocus-hsw:
+      label: [skip, flakes]
       checksum: 884096b6dd40e2f500d0837501287fac
+      text: "https://gitlab.freedesktop.org/mesa/mesa/-/issues/7158"
   unvanquished/unvanquished-ultra.trace:
     crocus-g41:
       label: [unsupported]