ci: remove stale target/ cache that caused E0152 duplicate lang item

target-main cache key doesn't invalidate on Cargo.lock changes, so
pre-fix serde_core compiled artifacts were restored into test builds,
reintroducing duplicate alloc lang items after the serde fork fix.

.gitlab-ci.yml

@@ -45,20 +45,14 @@ default:
     - rustup show
     - rustc --version && cargo --version
   cache:
-    - key:
-        files:
-          - Cargo.lock
-          - rust-toolchain.toml
-      paths:
-        - .cargo/registry/
-        - .cargo/git/
-      policy: pull-push
-    - key: "target-$CI_COMMIT_REF_SLUG"
-      paths:
-        - target/
-      policy: pull-push
-      fallback_keys:
-        - "target-$CI_DEFAULT_BRANCH"
+    key:
+      files:
+        - Cargo.lock
+        - rust-toolchain.toml
+    paths:
+      - .cargo/registry/
+      - .cargo/git/
+    policy: pull-push
   interruptible: true
 
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