Fix release building

What?
=====

GitHub Actions updated how environment variables need to be managed;
this was caught in
https://github.com/thoughtbot/complexity/runs/1534915774?check_suite_focus=true.

.github/workflows/release.yml

@@ -23,7 +23,7 @@ jobs:
           # Apparently, this is the right way to get a tag name. Really?
           #
           # See: https://github.community/t5/GitHub-Actions/How-to-get-just-the-tag-name/m-p/32167/highlight/true#M1027
-          echo "::set-env name=COMPLEXITY_VERSION::${GITHUB_REF#refs/tags/}"
+          echo "COMPLEXITY_VERSION=${GITHUB_REF#refs/tags/}" >> $GITHUB_ENV
           echo "version is: ${{ env.COMPLEXITY_VERSION }}"
 
       - name: Create GitHub release
@@ -93,9 +93,9 @@ 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 "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: |
@@ -113,10 +113,10 @@ jobs:
       shell: bash
       run: |
         release_upload_url="$(cat artifacts/release-upload-url)"
-        echo "::set-env name=RELEASE_UPLOAD_URL::$release_upload_url"
+        echo "RELEASE_UPLOAD_URL=$release_upload_url" >> $GITHUB_ENV
         echo "release upload url: $RELEASE_UPLOAD_URL"
         release_version="$(cat artifacts/release-version)"
-        echo "::set-env name=RELEASE_VERSION::$release_version"
+        echo "RELEASE_VERSION=$release_version" >> $GITHUB_ENV
         echo "release version: $RELEASE_VERSION"
 
     - name: Build release binary
@@ -136,7 +136,7 @@ jobs:
 
         cp "target/${{ matrix.target }}/release/complexity" "$staging/"
         tar czf "$staging.tar.gz" "$staging"
-        echo "::set-env name=ASSET::$staging.tar.gz"
+        echo "ASSET=$staging.tar.gz" >> $GITHUB_ENV
 
     - name: Upload release archive
       uses: actions/upload-release-asset@v1.0.1