ci: update to GITHUB_ENV

Apparently ::set-env has been completely disabled. Sigh.

.github/workflows/ci.yml

@@ -108,9 +108,9 @@ jobs:
       if: matrix.target != ''
       run: |
         cargo install 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 "CARGO=cross" >> $GITHUB_ENV
+        echo "TARGET_FLAGS=--target ${{ matrix.target }}" >> $GITHUB_ENV
+        echo "TARGET_DIR=./target/${{ matrix.target }}" >> $GITHUB_ENV
 
     - name: Show command used for Cargo
       run: |