MNT: Don't use deprecated functions in GitHub Actions

Apparently these have been changed for security reasons:
https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

.github/workflows/docs.yml

@@ -98,7 +98,7 @@ jobs:
 
     - name: Enable linkchecker for PRs
       if: ${{ github.event_name == 'pull_request' && matrix.check-links == true }}
-      run: echo "::set-env name=LINKCHECKER::linkcheck"
+      run: echo "LINKCHECKER=linkcheck" >> $GITHUB_ENV
 
     - name: Build docs
       run: |
@@ -120,7 +120,7 @@ jobs:
     # branch that's not master (which is confined to n.nn.x above) or on a tag.
     - name: Set doc version
       if: ${{ github.event_name != 'push' || !contains(github.ref, 'master') }}
-      run: echo "::set-env name=DOC_VERSION::v$(python -c 'import metpy; print(metpy.__version__.rsplit(".", maxsplit=2)[0])')"
+      run: echo "DOC_VERSION=v$(python -c 'import metpy; print(metpy.__version__.rsplit(".", maxsplit=2)[0])')" >> $GITHUB_ENV
 
     - name: Upload to GitHub Pages
       if: ${{ github.event_name != 'pull_request' && matrix.experimental == false }}

.github/workflows/linting.yml

@@ -25,7 +25,7 @@ jobs:
           curl -sfL \
             https://github.com/reviewdog/reviewdog/raw/master/install.sh | \
               sh -s -- -b $HOME/bin
-          echo ::add-path::$HOME/bin
+          echo "$HOME/bin" >> $GITHUB_PATH
 
       - name: Run flake8
         env: