Update System.Memory package only for source build (#18188)

Co-authored-by: Kevin Ransom (msft) <codecutter@hotmail.com>

Author: 0101

eng/Versions.props

@@ -81,7 +81,8 @@
     <SystemComponentModelCompositionVersion>$(SystemPackageVersionVersion)</SystemComponentModelCompositionVersion>
     <SystemCompositionVersion>$(SystemPackageVersionVersion)</SystemCompositionVersion>
     <SystemDiagnosticsDiagnosticSourceVersion>$(SystemPackageVersionVersion)</SystemDiagnosticsDiagnosticSourceVersion>
-    <SystemMemoryVersion>4.6.0</SystemMemoryVersion>
+    <SystemMemoryVersion Condition="'$(DotNetBuildSourceOnly)' != 'true'">4.5.5</SystemMemoryVersion>
+    <SystemMemoryVersion Condition="'$(DotNetBuildSourceOnly)' == 'true'">4.6.0</SystemMemoryVersion>
     <SystemReflectionEmitVersion>4.7.0</SystemReflectionEmitVersion>
     <SystemReflectionMetadataVersion>$(SystemPackageVersionVersion)</SystemReflectionMetadataVersion>
     <SystemRuntimeCompilerServicesUnsafeVersion>6.1.0</SystemRuntimeCompilerServicesUnsafeVersion>