Fix buildroot rootfs naming for riscv64 arch

[tollsimy]

This PR fixes arch naming for riscv64 platform arch for buildroot rootfs.

Steps to reproduce the bug:
kernelci-pipeline/config/platforms.yaml:

...

platforms:

  qemu:
    base_name: qemu
    arch: riscv64
    boot_method: qemu
    mach: qemu
    context:
      arch: riscv64
      cpu: rv64
      guestfs_interface: virt

...

When a test job is on this platform the rootfs url would be similar to the the following:
http://storage.kernelci.org/images/rootfs/buildroot/buildroot-baseline/20230421.0/riscv64/rootfs.cpio.gz
when instead the correct url is:
http://storage.kernelci.org/images/rootfs/buildroot/buildroot-baseline/20230421.0/riscv/rootfs.cpio.gz

[nuclearcat]

Thanks!