ci: bump to 2021.2.18f1 and remove custom image

Author: jsm174

.github/workflows/build.yml

@@ -67,8 +67,6 @@ jobs:
       - uses: game-ci/unity-test-runner@main
         id: test
         with:
-          unityVersion: '2021.2.16f1'
-          customImage: 'unityci/editor:2021.2.16f1-linux-il2cpp-1'
           projectPath: VisualPinball.Unity/VisualPinball.Unity.Test/TestProject~
           artifactsPath: VisualPinball.Unity/VisualPinball.Unity.Test/TestProject~/artifacts
           testMode: all

VisualPinball.Unity/VisualPinball.Unity.Test/TestProject~/ProjectSettings/ProjectVersion.txt

@@ -1,2 +1,2 @@
-m_EditorVersion: 2021.2.16f1
-m_EditorVersionWithRevision: 2021.2.16f1 (559fc0ec6670)
+m_EditorVersion: 2021.2.18f1
+m_EditorVersionWithRevision: 2021.2.18f1 (0c6e675195cf)