Only run benchmark bot on actual code changes

[EwoutH]

Only run if either an Python file has changed or if the benchmark workflow itself is changed.

It can be more fine-grained, but this should catch most docs-only cases.

[github-actions]

Performance benchmarks:

Model	Size	Init time [95% CI]	Run time [95% CI]
Schelling	small	🔵 -0.6% [-0.8%, -0.4%]	🔵 -0.1% [-0.4%, +0.1%]
Schelling	large	🔵 -1.2% [-1.6%, -0.8%]	🔵 -1.7% [-2.8%, -0.4%]
WolfSheep	small	🔵 -2.2% [-2.5%, -1.9%]	🔵 -0.7% [-0.9%, -0.6%]
WolfSheep	large	🔵 -3.5% [-7.1%, -0.7%]	🔵 -2.0% [-3.3%, -0.8%]
BoidFlockers	small	🔵 +2.2% [+1.9%, +2.5%]	🔵 +0.8% [-0.0%, +1.7%]
BoidFlockers	large	🔵 +2.6% [+1.9%, +3.3%]	🔵 +0.1% [-0.2%, +0.3%]