Toolchain: Actually disable vcpkg metrics

[ADKaster]

In the move to a python version of this script, I didn't notice that running the bootstrap script in shell mode precluded it from actually accepting the -disableMetrics argument.

Existing vcpkg installs can be un-metrics'd by re-running the bootstrap script with the disable argument, or by adding a vcpkg.disable-metrics file in $VCPKG_ROOT

thanks to @BenWiederhake for pointing this out 😅