Most users will be building with Xcode Clang on macOS anyway, as our
build scripts default to the system compiler if it's new enough. We
already have an upstream Clang-based workflow on Linux, so we won't lose
any compiler coverage by switching to Apple Clang on macOS.
This should help us avoid build breakages like #186.
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.
