Avoid launching as admin when NSIS installer ends.

The Bitcoin Core NSIS script runs with elevated privileges.  Unfortunately, this means that it launches Bitcoin Core itself with elevated privileges when the user chooses to launch Bitcoin Core at the end of the installation procedure.  This commit works around the issue by having explorer.exe launch Bitcoin Core.  Seems to be a similar approach to what http://nsis.sourceforge.net/ShellExecAsUser_plug-in does, but without a plugin.

h/t to "UK" at https://mdb-blog.blogspot.se/2013/01/nsis-lunch-program-as-user-from-uac.html?showComment=1410158039989#c2463780017054126736 for the sample code.

Fixes #7990.

share/setup.nsi.in

@@ -20,7 +20,8 @@ SetCompressor /SOLID lzma
 !define MUI_STARTMENUPAGE_REGISTRY_KEY ${REGKEY}
 !define MUI_STARTMENUPAGE_REGISTRY_VALUENAME StartMenuGroup
 !define MUI_STARTMENUPAGE_DEFAULTFOLDER "@PACKAGE_NAME@"
-!define MUI_FINISHPAGE_RUN $INSTDIR\@BITCOIN_GUI_NAME@@EXEEXT@
+!define MUI_FINISHPAGE_RUN "$WINDIR\explorer.exe"
+!define MUI_FINISHPAGE_RUN_PARAMETERS $INSTDIR\@BITCOIN_GUI_NAME@@EXEEXT@
 !define MUI_UNICON "${NSISDIR}\Contrib\Graphics\Icons\modern-uninstall.ico"
 !define MUI_UNWELCOMEFINISHPAGE_BITMAP "@abs_top_srcdir@/share/pixmaps/nsis-wizard.bmp"
 !define MUI_UNFINISHPAGE_NOAUTOCLOSE