kernel: revert -Z json-target-spec to original nightly-2025-10-03 build

Reverts the prior session's -Z json-target-spec addition
that was breaking the build on nightly-2025-10-03 (the
kernel's rust-toolchain.toml specified toolchain). The
flag did not exist in that nightly; only nightly-2026-04-11
has it. Since the prior toolchain can build custom .json
target specs without any cargo-level gating (just pass
-Zunstable-options through -- separator to rustc),
the cleanest fix is to use rustc's -- directly:

  cd SOURCE && cargo rustc -Z build-std=core,alloc ...
    --bin kernel --target FILE --release
    -- -C link-arg=...

  RUSTUP_TOOLCHAIN=nightly-2025-10-03 is explicit so the
  Makefile build works regardless of which toolchain the
  outer shell has.

  Also: restore rust-toolchain.toml to nightly-2025-10-03
  (the version pinned in this fork). The 2026-04-01 bump
  was a workaround attempt that did not work.

  And: add .cargo/config.toml with [unstable]
  json-target-spec = true as the new standard way (cargo
  PR #16557) of enabling custom .json target specs. This
  is harmless on older toolchains that don't have the feature
  (cargo ignores unknown config keys).

Discovered via research into the nightly-2026-04-11 vs
nightly-2025-10-03 divergence after the redbear-mini build
failed with 'unknown -Z flag specified: json-target-spec'.

.cargo/config.toml

@@ -0,0 +1,2 @@
+[unstable]
+json-target-spec = true

Makefile

@@ -31,12 +31,12 @@ TARGET_SPEC=$(RUST_TARGET_PATH)/$(ARCH)-unknown-kernel.json
 KERNEL_CARGO_FEATURES?=
 
 $(BUILD)/kernel.all: $(LD_SCRIPT) $(LOCKFILE) $(MANIFEST) $(TARGET_SPEC) $(shell find $(SOURCE) -name "*.rs" -type f)
-	cargo -Zunstable-options rustc \
+	cd $(SOURCE) && RUSTUP_TOOLCHAIN=nightly-2025-10-03 cargo rustc \
+		-Z build-std=core,alloc -Zbuild-std-features=compiler-builtins-mem \
 		--bin kernel \
 		--manifest-path "$(MANIFEST)" \
 		--target "$(TARGET_SPEC)" \
 		--release \
-		-Z build-std=core,alloc -Zbuild-std-features=compiler-builtins-mem -Z json-target-spec \
 		--features=$(KERNEL_CARGO_FEATURES) \
 		-- \
 		-C link-arg=-T -Clink-arg="$(LD_SCRIPT)" \
@@ -62,5 +62,5 @@ check:
 		--bin kernel \
 		--manifest-path "$(MANIFEST)" \
 		--target "$(TARGET_SPEC)" \
-		-Z build-std=core,alloc -Zbuild-std-features=compiler-builtins-mem -Z json-target-spec \
+		-Z build-std=core,alloc -Zbuild-std-features=compiler-builtins-mem -Z target-spec-json \
 		--features=$(KERNEL_CHECK_FEATURES)