ci: fix api diff job

Author: robjtede

.github/workflows/lint.yml

@@ -75,10 +75,10 @@ jobs:
 
       - uses: actions/checkout@v4
 
-      - name: Install Rust (nightly-2024-09-10)
-        uses: actions-rust-lang/[email protected].1
+      - name: Install Rust (${{ vars.RUST_VERSION_API_DIFF }})
+        uses: actions-rust-lang/[email protected].0
         with:
-          toolchain: nightly-2024-09-10
+          toolchain: ${{ vars.RUST_VERSION_API_DIFF }}
 
       - uses: taiki-e/[email protected]
         with: