[BUG] Check env in benchmarking script

[colin-ho]

Using ctx.get_or_create_runner in benchmarking warmup code / metrics builder causes subsequent ray.inits to crash. Just check the DAFT_RUNNER environment var instead, which should be set.

Tested:

• local -> https://github.com/Eventual-Inc/daft-benchmarking/actions/runs/11838323155
• remote -> https://github.com/Eventual-Inc/daft-benchmarking/actions/runs/11838783067

[codspeed-hq]

CodSpeed Performance Report

Merging #3297 will degrade performances by 47.65%

_{Comparing colin/fix-benchmarking-script (1d52ee2) with main (711e862)}

Summary

❌ 2 regressions
✅ 15 untouched benchmarks

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

Benchmarks breakdown

	Benchmark	main	colin/fix-benchmarking-script	Change
❌	test_iter_rows_first_row[100 Small Files]	269 ms	373.6 ms	-28%
❌	test_show[100 Small Files]	22.3 ms	42.5 ms	-47.65%

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.50%. Comparing base (711e862) to head (1d52ee2).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3297      +/-   ##
==========================================
- Coverage   77.50%   77.50%   -0.01%     
==========================================
  Files         666      666              
  Lines       81335    81333       -2     
==========================================
- Hits        63041    63036       -5     
- Misses      18294    18297       +3     

see 3 files with indirect coverage changes

[colin-ho]

The ray runner gets initialized in get_or_create_runner(), which calls ray.init, so the subsequent ray.init errors.