ci: github: Update for deprecation of add-path

Github has deprecated add-path, so update the workflows that use it to
the new method of setting the PATH.

[Backport of fabf403]

Signed-off-by: Kumar Gala <[email protected]>

Author: galak

.github/workflows/doc-build.yml

@@ -12,7 +12,7 @@ jobs:
     steps:
     - name: Update PATH for west
       run: |
-        echo "::add-path::$HOME/.local/bin"
+        echo "$HOME/.local/bin" >> $GITHUB_PATH
 
     - name: checkout
       uses: actions/checkout@v2