Windows installer: restore InstallScope setting.

This reverts commit 0615767224
("Windows installer: remove explicit InstallScope setting"), albeit
with different comments.

The original change worked around a Windows security vulnerability
(CVE-2023-21800), but also resulted in a rather broken installer.

windows/installer.wxs

@@ -121,6 +121,16 @@
       Language="1033" Codepage="1252" Version="$(var.Winver)">
 
     <!--
+        We force the install scope to perMachine, largely because I
+        don't really understand how to make it usefully switchable
+        between the two. If anyone is a WiX expert and does want to
+        install PuTTY locally in a user account, I hope they'll send a
+        well explained patch!
+
+        (Leaving InstallScope unset produces an installer that
+        superficially appears to work in some cases, but often fails
+        to do some of its work.)
+
         $(var.Puttytextver) is again defined on the candle command
         line, and describes the version of PuTTY in human-readable
         form, e.g. "PuTTY 0.67" or "PuTTY development snapshot [foo]".
@@ -129,7 +139,8 @@
              Description="$(var.Puttytextver) installer"
              Manufacturer="Simon Tatham"
              InstallerVersion="$(var.InstallerVersion)" Languages="1033"
-             Compressed="yes" SummaryCodepage="1252" />
+             Compressed="yes" SummaryCodepage="1252"
+             InstallScope="perMachine" />
 
     <!--
         Permit installing an arbitrary one of these PuTTY installers