We need to avoid using vcpkg's pkg-config on non-x86_64 platforms,
because they do very strange things to the default paths.
On Asahi Linux and other 16 KiB page distros, the user must also provide
a properly compiled version of GN.
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.
