ci: Use separate cache for subplot job

- Some subplot tests require a virtual card. Those are not always
    available on a system, so subplot uses the
    tests/virtual-card-available file to note whether a virtual card is
    available, and only generates those tests if it is.

  - That file is included in the cache, so the cache should not be
    shared.

.gitlab-ci.yml

@@ -144,8 +144,10 @@ subplot:
   cache:
     # inherit all general cache settings
     <<: *general_cache_config
-    # override the key
-    key: "rust-latest"
+    # subplot uses tests/virtual-card-available to indicate that tests which use
+    # virtual cards should be created. The cache with this file should not be
+    # shared.
+    key: "subplot"
 
 run_cardtest_smartpgp:
   stage: virtual-test