fix(makechrootpkg): pass --noprepare when downloading sources

The first makepkg invocation is broken if $BUILDDIR is set in chroot's makepkg.conf, and especially if that $BUILDDIR happens to point to the same (persistent) directory that is later bind-mounted to /build.

This way, the results of running prepare() with $srcdir=A are reused when running build() with $srcdir=B and if the build system is non-trivial everything breaks.

Try to mitigate this by not running prepare() in the first invocation.