Add third_party/ to CI workflow paths

This adds the third_party/ directory to the list of paths that will
trigger a CI workflow when changed.

Not all updates to third-party dependencies will necessarily change
code, but we bump dependencies infrequently so there's not much of a
problem with triggering CI on every change to third_party/.

.github/workflows/ci.yml

@@ -9,6 +9,7 @@ on:
       - "desktop_version/src/**.cpp"
       - "desktop_version/src/**.c"
       - "desktop_version/src/**.h"
+      - "third_party/**"
       - ".github/workflows/ci.yml"
   pull_request:
     paths:
@@ -16,6 +17,7 @@ on:
       - "desktop_version/src/**.cpp"
       - "desktop_version/src/**.c"
       - "desktop_version/src/**.h"
+      - "third_party/**"
       - ".github/workflows/ci.yml"
 
 env: