fix(colcon-build-and-test): ignore error on colcon mixin step (#34)

Signed-off-by: Keisuke Shima <19993104+KeisukeShima@users.noreply.github.com>

colcon-build-and-test/action.yaml

@@ -48,7 +48,7 @@ runs:
 
     - name: Set up colcon-mixin
       run: |
-        colcon mixin add default https://raw.githubusercontent.com/colcon/colcon-mixin-repository/master/index.yaml
+        colcon mixin add default https://raw.githubusercontent.com/colcon/colcon-mixin-repository/master/index.yaml || true
         colcon mixin update default
       shell: bash