Fix NativeAOT publish failure on fi_FI culture (#98552) (#98601)

This culture uses `U+2212 : MINUS SIGN` instead of `-` for negative numbers which trips up msbuild when comparing the property.
Instead of using an intermediate property just inline the usage and use `Contains()` for better readability.

Fixes #98550

(cherry picked from commit c768315)

Author: akoeplinger

src/coreclr/nativeaot/BuildIntegration/Microsoft.DotNet.ILCompiler.SingleEntry.targets

@@ -5,8 +5,7 @@
     <_hostOS>$(NETCoreSdkPortableRuntimeIdentifier.SubString(0, $(NETCoreSdkPortableRuntimeIdentifier.LastIndexOf('-'))))</_hostOS>
 
     <_targetOS>$(RuntimeIdentifier.SubString(0, $(RuntimeIdentifier.LastIndexOf('-'))))</_targetOS>
-    <_indexOfPeriod>$(_targetOS.IndexOf('.'))</_indexOfPeriod>
-    <_targetOS Condition="'$(_indexOfPeriod)' &gt; -1">$(_targetOS.SubString(0, $(_indexOfPeriod)))</_targetOS>
+    <_targetOS Condition="$(_targetOS.Contains('.'))">$(_targetOS.SubString(0, $(_targetOS.IndexOf('.'))))</_targetOS>
     <_targetOS Condition="$(_targetOS.StartsWith('win'))">win</_targetOS>
 
     <!-- On non-Windows, determine _hostArchitecture from NETCoreSdkPortableRuntimeIdentifier -->