ci: update env for openvm

Signed-off-by: Gregory Edison <gregory.edison1993@gmail.com>

Author: greged93

.github/workflows/lint.yml

@@ -280,6 +280,8 @@ jobs:
           cache-on-failure: true
       - run: cargo install --locked --git http://github.com/openvm-org/openvm.git --tag v1.4.0-rc.2 cargo-openvm
       - name: verify openvm compatibility
+        env:
+          OPENVM_RUST_TOOLCHAIN: nightly-2025-07-30
         run: cargo openvm build --manifest-path crates/scroll/openvm-compat/Cargo.toml
 
   lint-success: