Update build.yml

Author: fire

.github/workflows/build.yml

@@ -74,7 +74,7 @@ jobs:
           name: FBX2glTF-windows.exe
           path: build/Release/FBX2glTF.exe
   build-ubuntu:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     steps:
       - name: Checkout
         uses: actions/checkout@v2