Setup UnGAC: treat 259 as success (#9325)

One of the top VS Setup failures is a 259 from our UnGAC, even though we
have a big try/catch that should make it never fail. Tell the Setup
engine that 259 is another kind of success to keep this nonblocking.

Fixes AB#1899796.

Co-authored-by: Matt Cavallari <mjcavallari-zz@users.noreply.github.com>

src/Package/Microsoft.Build.UnGAC/exe.swr

@@ -17,3 +17,8 @@ vs.installSize
 
 vs.payloads
     vs.payload source=$(BinDir)Microsoft.Build.UnGAC.exe
+
+vs.returnCodes
+  vs.returnCode type=success
+    exitCode=259
+    details="Suppress return-code 259 since this is a best-effort ."