ci: remove export

Author: 0xPoe

.github/workflows/main.yml

@@ -14,7 +14,6 @@ jobs:
         run: cd os && make env
       - name: Check user
         run: |
-        export USER_IMG="../usr/build/riscv64.img"
         cd usr/rust/ && cargo fmt -- --check && cargo clippy -- -D warnings
       - name: Check os
         run: cd os && cargo fmt -- --check && cargo clippy -- -D warnings