message: ci: Drop insecure add_path, migrate to $GITHUB_PATH
  Github has deprecated setting environment variables from STDOUT, so
  we'll adopt their new recommended approach:
diff: @@ -113,12 +113,12 @@ jobs:
  - name: Install mdBook
  # Install prebuilt binaries where possible to improve CI performance
  run: |
  - mkdir -p $HOME/mdbook
  - curl -L https://github.com/rust-lang/mdBook/releases/download/v$MDBOOK_VERSION/mdbook-v$MDBOOK_VERSION-x86_64-unknown-linux-gnu.tar.gz | tar xz -C $HOME/mdbook
  - echo "::add-path::${HOME}/mdbook/"
  - mkdir -p $HOME/mdbook-linkcheck
  - curl -L https://github.com/Michael-F-Bryan/mdbook-linkcheck/releases/download/v$MDBOOK_LINKCHECK_VERSION/mdbook-linkcheck-v$MDBOOK_LINKCHECK_VERSION-x86_64-unknown-linux-gnu.tar.gz | tar xz -C $HOME/mdbook-linkcheck
  - echo "::add-path::${HOME}/mdbook-linkcheck/"
  + mkdir -p "$HOME/mdbook"
  + curl -L "https://github.com/rust-lang/mdBook/releases/download/v$MDBOOK_VERSION/mdbook-v$MDBOOK_VERSION-x86_64-unknown-linux-gnu.tar.gz" | tar xz -C "$HOME/mdbook"
  + echo "$HOME/mdbook/" >> $GITHUB_PATH
  + mkdir -p "$HOME/mdbook-linkcheck"
  + curl -L "https://github.com/Michael-F-Bryan/mdbook-linkcheck/releases/download/v$MDBOOK_LINKCHECK_VERSION/mdbook-linkcheck-v$MDBOOK_LINKCHECK_VERSION-x86_64-unknown-linux-gnu.tar.gz" | tar xz -C "$HOME/mdbook-linkcheck"
  + echo "$HOME/mdbook-linkcheck/" >> $GITHUB_PATH
  - name: Install Javascript dependencies
  run: yarn install
  working-directory: book
  