nsis: Specify OutFile path only once

Previously, we would specify the makensis output file path twice:

  1. At the top of Makefile.am as BITCOIN_WIN_INSTALLER, and
  2. In share/setup.nsi.ini

This commit uses the -X flag of makensis to eliminate the need for the
second instance mentioned above, referring makensis directly to the
value of BITCOIN_WIN_INSTALLER

share/setup.nsi.in

@@ -51,7 +51,6 @@ Var StartMenuGroup
 !insertmacro MUI_LANGUAGE English
 
 # Installer attributes
-OutFile @abs_top_srcdir@/@PACKAGE_TARNAME@-@PACKAGE_VERSION@-win64-setup-unsigned.exe
 InstallDir $PROGRAMFILES64\Bitcoin
 CRCCheck on
 XPStyle on