tests/functional/riscv64: Use newer kernel for tuxrun boot

The published tuxrun kernel is 6.4 which predates the introduction of AIA support in Linux. Update it to a 6.11 kernel, the newest build on the tuxrun site. Boot times on a laptop are slightly longer but still reasonable: | v6.4.16 | v6.11.9 ---------------------|---------|-------- test_riscv64 | 4.82s | 5.56s ---------------------|---------|-------- test_riscv64_maxcpu | 4.94s | 5.30s Signed-off-by: Joel Stanley <joel@jms.id.au> Reviewed-by: Alistair Francis <alistair.francis@wdc.com> Reviewed-by: Daniel Henrique Barboza <daniel.barboza@oss.qualcomm.com> Message-ID: <20260617054034.1020724-3-joel@jms.id.au> Signed-off-by: Alistair Francis <alistair.francis@wdc.com>