win build: setup exe should install to 64-bit "Program Files"

setup exe used to install Electrum to `C:\Program Files (x86)\Electrum`,
but now as the binaries are amd64, we should install to `C:\Program Files\Electrum`.

With this change, based on my experiments, looks like if there is an existing install,
nsis will keep using that path, but if it's a fresh install, it will now use the 64-bit path.

follow-up fcc4e1d387

contrib/build-wine/electrum.nsi

@@ -19,7 +19,7 @@
   OutFile "dist/electrum-setup.exe"
 
   ;Default installation folder
-  InstallDir "$PROGRAMFILES\${PRODUCT_NAME}"
+  InstallDir "$PROGRAMFILES64\${PRODUCT_NAME}"
 
   ;Get installation folder from registry if available
   InstallDirRegKey HKCU "Software\${PRODUCT_NAME}" ""