benchmarks.yml: Install SciPy and use uv for pip install

[EwoutH]

Fixes the broken benchmark workflow.

• Install SciPy, since that's now a dependency
• Use uv for pip install, which is way faster

[quaquel]

looks good to me

[github-actions]

Performance benchmarks:

Model	Size	Init time [95% CI]	Run time [95% CI]
BoltzmannWealth	small	🔵 +2.5% [+1.7%, +3.3%]	🔵 +2.8% [+2.2%, +3.1%]
BoltzmannWealth	large	🔵 -0.8% [-1.3%, -0.3%]	🔵 -1.0% [-2.5%, +0.3%]
Schelling	small	🔵 -1.0% [-1.4%, -0.7%]	🔵 -0.6% [-0.8%, -0.5%]
Schelling	large	🔵 -0.6% [-1.0%, -0.1%]	🔵 +0.3% [-0.2%, +0.9%]
WolfSheep	small	🔵 -1.1% [-1.4%, -0.7%]	🔵 -0.8% [-1.0%, -0.7%]
WolfSheep	large	🔵 -0.8% [-1.1%, -0.5%]	🔵 -0.8% [-1.3%, -0.1%]
BoidFlockers	small	🔵 -2.6% [-3.4%, -1.8%]	🔵 -1.2% [-1.4%, -1.1%]
BoidFlockers	large	🔵 -1.7% [-2.2%, -1.4%]	🔵 -0.8% [-1.0%, -0.6%]