Improve cache handling in benchmark tests

[seddonym]

Prior to this, the benchmark would use the file cache if it was already there, giving misleading results.

[codspeed-hq]

CodSpeed Performance Report

Merging #171 will not alter performance

_{Comparing dont-cache-in-benchmark (350b603) with master (9df4577)}

Summary

✅ 2 untouched benchmarks
🆕 2 new benchmarks
⁉️ 1 (👁 1) dropped benchmarks

Benchmarks breakdown

	Benchmark	master	dont-cache-in-benchmark	Change
👁	test_build_django	223.6 ms	N/A	N/A
🆕	test_build_django_from_cache	N/A	225.3 ms	N/A
🆕	test_build_django_uncached	N/A	6.2 s	N/A