Add comments explaining why these targets are disabled

Clean up the run-build script

run-build.ps1

@@ -24,10 +24,7 @@ if ($PgoInstrument) {
 }
 
 if ($Sign) {
-  $Parameters = "$Parameters -sign /p:SignCoreSdk=true"
-
-  # Workaround https://github.com/dotnet/arcade/issues/1776
-  $WarnAsError = $false
+  $Parameters = "$Parameters -sign"
 }
 
 if ($Pack) {

src/redist/targets/Crossgen.targets

@@ -1,5 +1,6 @@
 <Project>
   <!-- Crossgen is currently not supported on the s390x, ppc64le architecture as using mono instead of CoreCLR. -->
+  <!-- Disable the cross-gen step in the local dev build, only do when -pack is specified to speed up dev builds -->
   <Target Name="CrossgenLayout"
           Condition="'$(DISABLE_CROSSGEN)' == '' AND '$(Architecture)' != 's390x' AND '$(Architecture)' != 'ppc64le' AND '$(PackInstaller)' == 'true'"
           DependsOnTargets="SetSdkBrandingInfo">

src/redist/targets/GenerateArchives.targets

@@ -1,4 +1,5 @@
 <Project>
+    <!-- Disable the GenerateArchives step in the local dev build, only do when -pack is specified to speed up dev builds -->
   <Target Name="GenerateArchives"
           Condition="'$(PackInstaller)' == 'true'"
           DependsOnTargets="GenerateLayout;GetCurrentRuntimeInformation"