fix: make nvattest builds reproducible

[kvinwang]

Summary

• pin the generated regorus FFI Cargo dependency graph used by NVIDIA's attestation SDK
• install the reviewed lockfile after CMake FetchContent populates regorus
• fail the build if Cargo attempts to rewrite the lockfile

Root cause

Two independent clean dstack OS builds produced different /usr/bin/nvattest and /usr/lib/libnvat.so.1.2.2. Their executable code, read-only data, relocations and writable data were identical; only dynamic-string ordering, debug links and GNU build IDs differed.

regorus/bindings/ffi does not commit a Cargo.lock, so clean builds performed time-dependent dependency resolution. The observed builds selected syn 3.0.2 and 3.0.3 respectively. That changed proc-macro build inputs and ultimately ELF string-table ordering/build IDs.

Verification

Two independent mc:prod:nvattest cleansstate rebuilds after this fix produced byte-identical files:

• nvattest: 32c954ab703d37b3ab6594f54997083ac5f2db4b3f82b372c01eef1abb3d98a1
• libnvat.so.1.2.2: 2c8c1104fa0ab4f68dd96319d2040d6a303f00055cd9026c9637948ac3982ffd

Rebuilding both production rootfs images then produced byte-identical outputs:

• CPIO: 55e0121dfa0f44a3a5dfba69dd7d49fa0c101dcff8156eb2826d1a1d237fc991
• squashfs: 53616e21428f956f1782597ced25bdbe3233e37a0575ad41f566b69f90f8608f
• squashfs.verity: 8ea8229ec9fd8fb105ebb4e327a5d71c83c1571ea28ecf1346a4d31bdbb51921

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