Update build.yml

Explicitly ask for specific versions of Windows, Ubuntu, and MacOS to prevent future "surprise" issues.

Also, now using a slightly older Ubuntu version to allow people to still use 18.04 with the auto generated package.

.github/workflows/build.yml

@@ -8,7 +8,7 @@ on:
 jobs:
   buildlinux:
     name: Linux x64
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
 
     steps:
       - name: Prepare Environment
@@ -40,7 +40,7 @@ jobs:
       
   buildmacos:
     name: macOS x64
-    runs-on: macos-latest
+    runs-on: macos-10.15
 
     steps:
       - name: Prepare macOS Environment
@@ -69,7 +69,7 @@ jobs:
           
   buildwindows:
     name: Windows x64
-    runs-on: windows-latest
+    runs-on: windows-2019
 
     steps:      
       - name: Prepare Qt Libraries