ci: add export

Author: Rustin170506

.github/workflows/main.yml

@@ -15,7 +15,7 @@ jobs:
       - name: Check user
         run: cd usr/rust/ && cargo fmt -- --check && cargo clippy -- -D warnings
       - name: Check os
-        run: cd os && cargo fmt -- --check && cargo clippy -- -D warnings
+        run: export USER_IMG="../usr/build/riscv64.img" && cd os && cargo fmt -- --check && cargo clippy -- -D warnings
   build:
     runs-on: ubuntu-latest
     steps: