Use borrowed build paths for objdiff build setup

[encounter]

Summary

• update objdiff job build path handling to borrow project and object paths instead of cloning buffers
• ensure build path validation works with build_base: false scenarios without requiring owned paths

Testing

• cargo +nightly fmt --all
• cargo +nightly clippy --all-targets --all-features --workspace -- -D warnings (fails: objdiff-wasm build script cannot fetch wit/deps due to invalid TLS certificate)

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Codex Task