tests: kernel: interrupt: Do not test interrupt nesting on RISC-V

This commit disables the nested interrupt test for the RISC-V platform,
as interrupt nesting is not supported on the current RISV-C
architecture port.

Furthermore, the current `trigger_irq` implementation for RISC-V is
mostly incorrect and cannot be used, so there is no point in leaving
that in the codebase (see #23593).

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>