Pass P2E build paths into the Bebop Nix shell

[daiyongyuan]

Summary

• Pass VSRC_PATH and OUT_PATH into the isolated Nix development shell used by the P2E bitstream build.
• Run from the Bebop directory so Nix loads its development environment and Spike source checkout.

Validation

• Confirmed the generated Pebble P2E runtime builds in the Bebop Nix environment.
• Ran the Pebble fw_payload-lenet workload with --diff through bbdev in the root Nix environment; LeNet Inference PASS and Bank DiffTest passed.