Fix benchmarks to run with older commits

[mraspaud]

This PR makes downloading the pyspectral luts faster.
It also adds some asumption to the ahi benchmarks to allow running on older commits.

[codecov]

Codecov Report

Merging #1715 (8a19f82) into main (1289aa3) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1715   +/-   ##
=======================================
  Coverage   92.44%   92.44%           
=======================================
  Files         258      258           
  Lines       38279    38279           
=======================================
  Hits        35388    35388           
  Misses       2891     2891           

Flag	Coverage Δ
behaviourtests	4.84% <ø> (ø)
unittests	92.98% <ø> (ø)

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[coveralls]

Coverage remained the same at 92.936% when pulling 8a19f82 on mraspaud:fix-benchmarks-speed into 1289aa3 on pytroll:main.