CI: Pin Emscripten to 3.1.39

Due to godotengine#82865, newer versions can't be used for dlink-enabled Web builds.
This isn't a problem for CI which doesn't use dlink, but it's clearer for
users if our CI version matches the one we use for official builds.

(cherry picked from commit 51aff13)

.github/workflows/web_builds.yml

@@ -7,7 +7,7 @@ env:
   # Used for the cache key. Add version suffix to force clean build.
   GODOT_BASE_BRANCH: '4.1'
   SCONSFLAGS: verbose=yes warnings=extra werror=yes debug_symbols=no
-  EM_VERSION: 3.1.45
+  EM_VERSION: 3.1.39
   EM_CACHE_FOLDER: "emsdk-cache"
 
 concurrency: