Switch to label.N form for pre-release label (dotnet/core-setup#8661)

* Switch to label.N form for pre-release label
In order to facilitate better preview sorting, switch to label.N form for the pre-release label.


Commit migrated from https://github.com/dotnet/core-setup/commit/6c6a130ba15615a4b589b6e2d8120f4f30f8275f

src/installer/pkg/Directory.Build.targets

@@ -14,11 +14,14 @@
       Condition="
         '$(ReleaseBrandSuffix)' == '' and
         '$(PreReleaseVersionLabel)' != '' and
-        $(PreReleaseVersionLabel.StartsWith('preview'))">
-      <!-- Convert 'preview7' to 'Preview 7'. -->
+        '$(PreReleaseVersionIteration)' != ''">
+      <!-- Convert 'preview.7' to 'Preview 7'.
+          'preview' will come from the pre-release version iteration and the numeric value
+          will be the PreReleaseVersionIteration. -->
+
       <ReleaseBrandSuffix>$(PreReleaseVersionLabel.Substring(0,1).ToUpperInvariant())</ReleaseBrandSuffix>
-      <ReleaseBrandSuffix>$(ReleaseBrandSuffix)$(PreReleaseVersionLabel.Substring(1,6))</ReleaseBrandSuffix>
-      <ReleaseBrandSuffix>$(ReleaseBrandSuffix) $(PreReleaseVersionLabel.Substring(7))</ReleaseBrandSuffix>
+      <ReleaseBrandSuffix>$(ReleaseBrandSuffix)$(PreReleaseVersionLabel.Substring(1))</ReleaseBrandSuffix>
+      <ReleaseBrandSuffix>$(ReleaseBrandSuffix) $(PreReleaseVersionIteration)</ReleaseBrandSuffix>
     </PropertyGroup>
 
     <PropertyGroup>