fix(fpga): support no-diff without specified ref-so

[klin02]

Previously we always use difftest_init(true, ) even with --no-diff, which requires us to specify ref-so path or NEMU_HOME.

This change replace it with difftest_init(args.enable_diff, ).

[github-actions]

Current result (nutshell) - 2026-04-08 21:38

Instance	Module	Total LUTs	Logic LUTs	LUTRAMs	SRLs	FFs	RAMB36	RAMB18	URAM	DSP Blocks
U_CPU_TOP	SimTop_wrapper	125502 (3.1%)	116198	9304	0	93057	17	13	4	16
nutcore	NutCore	16587 (0.4%)	16427	160	0	20989	17	9	0	16
difftest_host	HostEndpoint	25417 (0.6%)	16273	9144	0	32155	0	0	0	0
endpoint	GatewayEndpoint	81123 (2.0%)	81123	0	0	37454	0	0	0	0

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Archive (latest, 20260406)

Instance	Module	Total LUTs	Logic LUTs	LUTRAMs	SRLs	FFs	RAMB36	RAMB18	URAM	DSP Blocks
U_CPU_TOP	SimTop_wrapper	117632 (2.9%)	106436	11196	0	67653	17	13	4	16
nutcore	NutCore	16589 (0.4%)	16429	160	0	20989	17	9	0	16
difftest_host	HostEndpoint	25415 (0.6%)	16271	9144	0	16454	0	0	0	0
endpoint	GatewayEndpoint	73253 (1.8%)	71361	1892	0	27751	0	0	0	0