fix(fpga-nightly): track default branchs and fpga-release

[klin02]

1. This change rename the FPGA Nightly CI job for clarity.
2. The nightly flow now tracks the default branches of env-scripts and NutShell, ensuring it follows upstream changes by default.
3. FPGA regressions are executed using the fpga-release path to standardize operational workflows

[klin02]

Temporarily use PR instead of schedule for testing flow.

[klin02]

Test OK with Action https://github.com/OpenXiangShan/difftest/actions/runs/21022877819

[github-actions]

Current result (nutshell) - 2026-01-16 21:09

Instance	Module	Total LUTs	Logic LUTs	LUTRAMs	SRLs	FFs	RAMB36	RAMB18	URAM	DSP Blocks
U_CPU_TOP	SimTop_wrapper	117485 (2.9%)	106165	11320	0	67655	17	13	4	16
nutcore	NutCore	16539 (0.4%)	16379	160	0	20993	17	9	0	16
difftest_host	HostEndpoint	25415 (0.6%)	16271	9144	0	16454	0	0	0	0
endpoint	GatewayEndpoint	73081 (1.8%)	71065	2016	0	27749	0	0	0	0

---

Archive (latest, 20251217)

Instance	Module	Total LUTs	Logic LUTs	LUTRAMs	SRLs	FFs	RAMB36	RAMB18	URAM	DSP Blocks
U_CPU_TOP	SimTop_wrapper	117337 (2.9%)	106009	11328	0	67301	17	13	4	16
nutcore	NutCore	16531 (0.4%)	16371	160	0	20993	17	9	0	16
difftest_host	HostEndpoint	25271 (0.6%)	16119	9152	0	16076	0	0	0	0
endpoint	GatewayEndpoint	73082 (1.8%)	71066	2016	0	27773	0	0	0	0