Fix rule syntax

static-code-analysis.yml

@@ -15,7 +15,7 @@ run_static_code_analysis:
   script:
     - "static_code_analysis.sh"
   rules:
-    - if: ${SOURCE_FOLDER}
+    - if: $SOURCE_FOLDER
       when: always
   artifacts:
     when: on_failure
@@ -32,5 +32,5 @@ run_shell_checks:
     - echo "Running shellcheck ${SHELLCHECK_OPTIONS} ${SHELL_SRC}"
     - "shellcheck ${SHELLCHECK_OPTIONS} ${SHELL_SRC}"
   rules:
-    - if: ${SHELL_SRC}
+    - if: $SHELL_SRC
       when: always