ci: Move LINT_LOG variable out of global section

It is only used by the eslint job, so better define it there.

While at it, make sure the variable is quoted as that's considered
good practice (even when safe to use unquoted as in this case).

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/367>

.gitlab-ci.yml

@@ -31,7 +31,6 @@ default:
 variables:
   FDO_UPSTREAM_REPO: GNOME/gnome-shell-extensions
   MESON_BUILD_DIR: build
-  LINT_LOG: "eslint-report.xml"
 
 workflow:
   rules:
@@ -109,14 +108,16 @@ js_check:
 eslint:
   stage: review
   <<: *prereview_req
+  variables:
+    LINT_LOG: "eslint-report.xml"
   script:
     - export NODE_PATH=$(npm root -g)
-    - ./.gitlab-ci/run-eslint --output-file ${LINT_LOG} --format junit --stdout
+    - ./.gitlab-ci/run-eslint --output-file "$LINT_LOG" --format junit --stdout
   artifacts:
     paths:
-      - ${LINT_LOG}
+      - "$LINT_LOG"
     reports:
-      junit: ${LINT_LOG}
+      junit: "$LINT_LOG"
 
 potfile_js_check:
   stage: review