[ci] Use prebuilt containers to build and test iPXE

[ci] Use prebuilt containers to build and test iPXE

Use the prebuilt containers from https://github.com/ipxe/ipxe-builder
to build BIOS, SBI, UEFI, and Linux userspace versions of iPXE for all
supported CPU architectures, and to run the Linux userspace test suite
(via valgrind or qemu as applicable).

This reduces the time taken for GitHub CI runs by around 80%, while
increasing the build coverage to include RISC-V SBI, RISC-V UEFI, and
LoongArch64 UEFI, and increasing the test coverage to include running
the Linux userspace test suite on all supported CPU architectures.

Signed-off-by: Michael Brown <mcb30@ipxe.org>