Fix task key computation

[zzstoatzz]

closes #13735

this PR removes our reliance on the absolute path of the source file, which caused persistent cache misses due to source code being cloned to tmp dirs

the original issue was reported on a 2.x version - if this looks good I can backport this to 2.x

[codspeed-hq]

CodSpeed Performance Report

Merging #14704 will degrade performances by 11.46%

_{Comparing task-key-computation (e7d8691) with main (1299f1a)}

Summary

❌ 1 regressions
✅ 4 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	task-key-computation	Change
❌	bench_task_decorator	434.7 µs	490.9 µs	-11.46%

[cicdw]

One suggestion to move more logic into your utility function

[cicdw]

Awesome, thank you!

[cicdw]

LGTM, and the Codspeed tests are still new so we can merge without forcing that one benchmark to pass right now, especially given the variance and how tiny the "slowdown" is.