OVMF: disable sourceDebug by default

`sourceDebug` enables a source-level debugger agent inside the UEFI code
that can be leveraged via a certain bus (USB or serial for example) to debug the platform.

See: https://github.com/tianocore/tianocore.github.io/wiki/SourceLevelDebugPkg for more information.
`sourceDebug` is almost something you never want when you debug UEFI in a virtual machine.

Debugging with GDB can be achieved via: https://github.com/tianocore/tianocore.github.io/wiki/How-to-debug-OVMF-with-QEMU-using-GDB.

pkgs/applications/virtualization/OVMF/default.nix

@@ -7,7 +7,9 @@
 , tpmSupport ? false
 , tlsSupport ? false
 , debug ? false
-, sourceDebug ? debug
+# Usually, this option is broken, do not use it except if you know what you are
+# doing.
+, sourceDebug ? false
 }:
 
 assert csmSupport -> seabios != null;