fix shell-check

Author: cospectrum

.github/workflows/safety.yml

@@ -59,7 +59,7 @@ jobs:
         with:
           submodules: true
       - run: |
-          echo "NIGHTLY=nightly-$(curl -s https://rust-lang.github.io/rustup-components-history/x86_64-unknown-linux-gnu/miri)" >> $GITHUB_ENV
+          echo "NIGHTLY=nightly-$(curl -s https://rust-lang.github.io/rustup-components-history/x86_64-unknown-linux-gnu/miri)" >> "$GITHUB_ENV"
       - name: Install ${{ env.NIGHTLY }}
         uses: dtolnay/rust-toolchain@master
         with: