more robust InstallPath (amending 4ce07be8ba17): self-directory may be odd temp dir produced by browser "Run" operation;

--- a/Admin/Windows/Installer/sfx.txt	Fri May 24 20:16:35 2019 +0200
+++ b/Admin/Windows/Installer/sfx.txt	Fri May 24 21:14:02 2019 +0200
@@ -1,6 +1,6 @@
 ;!@Install@!UTF-8!
 GUIFlags="64"
-InstallPath="%%S"
+InstallPath="%UserDesktop%"
 BeginPrompt="Unpack {ISABELLE_NAME}?"
 ExtractPathText="Target directory"
 ExtractTitle="Unpacking {ISABELLE_NAME} ..."