Fix Linux CI (#47)

* Change linux build command to conform to dotnet8 requirements

* Run CI on Pull Requests

.github/workflows/main.yml

@@ -4,6 +4,7 @@ on:
   push:
     paths-ignore:
       - 'README.md'
+  pull_request:
   workflow_dispatch:
 
 jobs:
@@ -15,7 +16,7 @@ jobs:
         configuration: [Release]
         include:
           - os: ubuntu-latest
-            COMMAND: Atomic.Gtk -p:PublishSingleFile=true -p:DebugType=embedded -r ubuntu.18.04-x64 --no-self-contained -o builds/ubuntu-latest -p:BuildPlatform=Gtk
+            COMMAND: Atomic.Gtk -p:PublishSingleFile=true -p:DebugType=embedded -r linux-x64 --no-self-contained -o builds/ubuntu-latest -p:BuildPlatform=Gtk
             ARTIFACT: builds/ubuntu-latest
             POSTBUILD: echo "nothing to do!"
           - os: macos-13