fix[duckdb]: build cpp.rs only if changed

[joseph-isaacs]

Only build rust files if they change. Add a CI check for this.

The check works: #6952

[codspeed-hq]

Merging this PR will improve performance by 18.36%

⚡ 2 improved benchmarks
✅ 1007 untouched benchmarks
⏩ 1515 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	bitwise_not_vortex_buffer_mut[128]	376.1 ns	317.8 ns	+18.36%
⚡	Simulation	bitwise_not_vortex_buffer_mut[1024]	535.6 ns	477.2 ns	+12.22%

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_{Comparing ji/fix-buildrs-rebuild (0640ec1) with develop (2a0d1b7)}

Footnotes

1. 1515 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩