CI: scripts: fix build of vtest regarding option -C

CI: scripts: fix build of vtest regarding option -C

On Linux, GNU make emits "w" at the beginning of the MAKEFLAGS
variable if -C is passed, which happens since vtest d6d228bcb3.
In fact it emits any of the command line flags without the leading
'-' in this case. gmake doesn't do that on BSD apparently. It's
documented under Options/Recursion in the GNU make doc. There's
also MFLAGS that could work but it does not contain the variables
definitions. So let's just avoid the -C that we don't really need.

This needs to be backported to stable versions.