ci: Changed standalone tests trigger on PRs to use only Ubuntu (#3432)

It was noted that after we switched macOS from x64 to Apple Silicon the
distribution times started taking up to 40m which is fine for Nightly
jobs but it's frustrating for PRs triggers.
We decided that coverage on Ubuntu is enough for PR trigger since all
other standalone desktop devices are being tested during Nightly.

## Backport
Port for `develop-2.0.0` branch is present in
#3431

Author: michalChrobot

.yamato/_triggers.yml

@@ -52,9 +52,9 @@ pull_request_trigger:
     - .yamato/_run-all.yml#run_all_project_tests_trunk
     # Run project EditMode and PLaymode project tests on minimum supported editor (2021.3 in case of NGOv1.X)
     - .yamato/_run-all.yml#run_all_project_tests_2021
-    # Run standalone test with mixture of parameters to make sure there are no obvious issues with most common platform (for example --fail-on-assert option is not present with package/project tests). We run 2 different combinations with different platform/editor/scripting backend.
-    - .yamato/desktop-standalone-tests.yml#desktop_standalone_test_testproject_mac_il2cpp_trunk
-    - .yamato/desktop-standalone-tests.yml#desktop_standalone_test_testproject_win_mono_2021.3
+    # Run standalone test. We run it only on Ubuntu since it's the fastest machine, and it was noted that for example distribution on macOS is taking 40m since we switched to Apple Silicon
+    # Coverage on other standalone machines is present in Nightly job so it's enough to not run all of them for PRs
+    - .yamato/desktop-standalone-tests.yml#desktop_standalone_test_testproject_ubuntu_il2cpp_2021.3
   triggers:
     cancel_old_ci: true
     pull_requests: