ci: fix public api diff job

Author: robjtede

.github/workflows/lint.yml

@@ -77,10 +77,10 @@ jobs:
       - name: Checkout PR branch
         uses: actions/checkout@v4
 
-      - name: Install Rust
-        uses: actions-rust-lang/setup-rust-toolchain@v1.10.1
+      - name: Install Rust (${{ vars.RUST_VERSION_API_DIFF }})
+        uses: actions-rust-lang/setup-rust-toolchain@v1.8.0
         with:
-          toolchain: nightly-2024-09-10
+          toolchain: ${{ vars.RUST_VERSION_API_DIFF }}
 
       - name: Install cargo-public-api
         uses: taiki-e/[email protected]