fix ordering to make sure that we only ever _write_ a default value after we've checked all possible user-provided values

Author: baronfel

src/Containers/packaging/build/Microsoft.NET.Build.Containers.targets

@@ -37,8 +37,8 @@
       <!-- The Container RID should default to the RID used for the entire build (to ensure things run on the platform they are built for), but the user knows best and so should be able to set it explicitly.
            For builds that have a RID, we default to that RID. Otherwise, we default to the Linux RID matching the architecture of the currently-executing SDK. -->
       <ContainerRuntimeIdentifier Condition="'$(ContainerRuntimeIdentifier)' == '' and '$(RuntimeIdentifier)' != ''">$(RuntimeIdentifier)</ContainerRuntimeIdentifier>
-      <ContainerRuntimeIdentifier Condition="'$(ContainerRuntimeIdentifier)' == '' and '$(ContainerRuntimeIdentifiers)' == ''">linux-$(NETCoreSdkPortableRuntimeIdentifier.Split('-')[1])</ContainerRuntimeIdentifier>
       <ContainerRuntimeIdentifiers Condition="'$(BuildingInsideVisualStudio)' != 'true' and '$(ContainerRuntimeIdentifiers)' == '' and '$(IsRidAgnostic)' != 'true'">$(RuntimeIdentifiers)</ContainerRuntimeIdentifiers>
+      <ContainerRuntimeIdentifier Condition="'$(ContainerRuntimeIdentifier)' == '' and '$(ContainerRuntimeIdentifiers)' == ''">linux-$(NETCoreSdkPortableRuntimeIdentifier.Split('-')[1])</ContainerRuntimeIdentifier>
 
       <_ContainerIsUsingMicrosoftDefaultImages Condition="'$(ContainerBaseImage)' == ''">true</_ContainerIsUsingMicrosoftDefaultImages>
       <_ContainerIsUsingMicrosoftDefaultImages Condition="'$(ContainerBaseImage)' != ''">false</_ContainerIsUsingMicrosoftDefaultImages>

src/Tests/Microsoft.NET.Build.Containers.IntegrationTests/EndToEndTests.cs

@@ -1276,7 +1276,7 @@ public void EndToEndMultiArch_Labels()
         // Run PublishContainer for multi-arch with ContainerGenerateLabels
         new DotnetCommand(
             _testOutput,
-            "build",
+            "publish",
             "/t:PublishContainer",
             "/p:RuntimeIdentifiers=\"linux-x64;linux-arm64\"",
             $"/p:ContainerBaseImage={DockerRegistryManager.FullyQualifiedBaseImageAspNet}",