doc/install/windows_source_install.md: with Smart App Control on, the MSYS2
installer never finishes, the MSYS2 shell does not open, and TeX Live cannot
be installed -- its installer brings an unsigned Perl. The paragraph quotes
both messages (the command line's and the Windows Security notice), and
Troubleshooting says to close the installer, switch Smart App Control off
and run it again. Found by performing the page on a pristine Windows 11.