GitHub actions: use ubuntu-20.04 instead of latest

Author: TheMostDiligent

.github/workflows/build-android.yml

@@ -4,7 +4,7 @@ on: [push, pull_request]
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     strategy:
       matrix:

.github/workflows/build-emscripten.yml

@@ -6,7 +6,7 @@ jobs:
   build:
     strategy:
       matrix:
-        os:         ["windows-latest", "ubuntu-latest", "macos-latest"]
+        os:         ["windows-latest", "ubuntu-20.04", "macos-latest"]
         build_type: ["Debug", "Release"]
 
         exclude:
@@ -18,7 +18,7 @@ jobs:
           - os:        "windows-latest"
             host_name: "Win10"
 
-          - os:        "ubuntu-latest"
+          - os:        "ubuntu-20.04"
             host_name: "Linux"
 
           - os:        "macos-latest"

.github/workflows/codeql.yml

@@ -18,7 +18,7 @@ jobs:
 
           - host_name:       "Linux"
             platform:        "Linux"
-            os:              "ubuntu-latest"
+            os:              "ubuntu-20.04"
             cmake_generator: "Ninja"
             cc:              "clang-12"
             cxx:             "clang++-12"