Also add a vcpkg command to ladybird.sh to ensure that vcpkg is setup,
and use a local binary cache for vcpkg build and install media to
avoid cluttering $XDG_CACHE_HOME.
And hook it into ladybird.sh for convenience. The script will set up
PATH and other environment variables automatically.
On CI, vcpkg is theoretically already installed on Linux machines, but
not with the right environment variables, and not on macOS. So this also
makes CI use this script to bootstrap vcpkg.
