try this

.gitlab-ci.yml

@@ -19,13 +19,13 @@ variables:
 .default-cache: &default-cache
   key: "default-cache-${CI_COMMIT_REF_SLUG}"
   paths:
-    - $APT_CACHE
-    - $R_LIBS_USER
+    - ${APT_CACHE}
+    - ${R_LIBS_USER}
 
 .content-cache: &content-cache
   key: "content-cache-${CI_COMMIT_REF_SLUG}"
   paths:
-    - $CACHE_DIR
+    - ${CACHE_DIR}
 
 default:
   before_script:
@@ -50,7 +50,7 @@ default:
         remotes::install_deps(dependencies = TRUE, upgrade = FALSE, quiet = FALSE)
       "
   cache:
-    - $GRAALVM_HOME
+    - ${GRAALVM_HOME}
     - <<: *default-cache