bootstrap: Fix cross-windows bootstrap with --system-only
When we're bootstrapping on Linux for cross-windows, we don't want "Windows" as a system bootstrapper. So we shouldn't be looping over all the system bootstrappers. We should just pick the one that matches the build distro. Same for the toolchain, because there can only be at most one that matches the target distro.
Fixes the issue found in https://gitlab.freedesktop.org/alatiera/gst-ci/-/jobs/5026702
Edited by Nirbheek Chauhan