CI: Set PROJ_LIB in the Benchmarks workflow to fix the proj_create_from_database error

[seisman]

Description of proposed changes

In the "Benchmarks" workflow (e.g., https://github.com/GenericMappingTools/pygmt/actions/runs/9001470390/job/24727777928), we can see an error like:

 ERROR 1: PROJ: proj_create_from_database: Open of /usr/share/miniconda/share/proj failed

which is not seen in the Tests workflow (e.g., https://github.com/GenericMappingTools/pygmt/actions/runs/9039830454/job/24843235152).

Currently, the "Benchmarks" workflow is not affected by the error and still passes, but it fails in #3193. So, we should fix the error.

It's unclear what's happening exactly, but the solution is simple. We just need to set the PROJ_LIB environment variable. xref: https://gis.stackexchange.com/questions/364421/how-to-make-proj-work-via-anaconda-in-google-colab/370360#370360

After this patch, the error disappears. See https://github.com/GenericMappingTools/pygmt/actions/runs/9039893228/job/24843381919

[codspeed-hq]

CodSpeed Performance Report

Merging #3241 will degrade performances by 11.31%

_{Comparing ci/benchmarks-proj (7e5b7da) with main (4fbf32f)}

Summary

❌ 1 regressions
✅ 98 untouched benchmarks

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

Benchmarks breakdown

	Benchmark	main	ci/benchmarks-proj	Change
❌	test_grdsample_dataarray_out	48.7 ms	54.9 ms	-11.31%