CI: fix set-env https://docs.github.com/en/free-pro-team@latest/actions/reference/workflow-commands-for-github-actions#environment-files

.github/workflows/ci.yml

@@ -79,9 +79,9 @@ jobs:
         # FIXME: to work around bugs in latest cross release, install master.
         # See: https://github.com/rust-embedded/cross/issues/357
         cargo install --git https://github.com/rust-embedded/cross
-        echo "::set-env name=CARGO::cross"
-        echo "::set-env name=TARGET_FLAGS::--target ${{ matrix.target }}"
-        echo "::set-env name=TARGET_DIR::./target/${{ matrix.target }}"
+        echo "cross" >> $CARGO
+        echo "--target ${{ matrix.target }}" >> $TARGET_FLAGS
+        echo "./target/${{ matrix.target }}" >> $TARGET_DIR
 
     - name: Show command used for Cargo
       run: |