Update deprecated GitHub add-path in workflows.

Author: ehuss

.github/workflows/contrib.yml

@@ -15,7 +15,7 @@ jobs:
       run: |
         mkdir mdbook
         curl -Lf https://github.com/rust-lang/mdBook/releases/download/v0.4.3/mdbook-v0.4.3-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=./mdbook
-        echo ::add-path::`pwd`/mdbook
+        echo `pwd`/mdbook >> $GITHUB_PATH
     - name: Deploy docs
       run: |
         cd src/doc/contrib

.github/workflows/main.yml

@@ -97,7 +97,7 @@ jobs:
     - run: |
         mkdir mdbook
         curl -Lf https://github.com/rust-lang/mdBook/releases/download/v0.3.7/mdbook-v0.3.7-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=./mdbook
-        echo ::add-path::`pwd`/mdbook
+        echo `pwd`/mdbook >> $GITHUB_PATH
     - run: cargo doc --no-deps
     - run: cd src/doc && mdbook build --dest-dir ../../target/doc
     - run: |