New betainc C implementation less precise than Scipy

[ricardoV94]

PyMC CI started failing in pymc-devs/pymc#7400

import scipy.special
import pytensor.tensor as pt

print(pt.betainc(100.0, 1.0, 0.1).eval())  # 0.0
print(scipy.special.betainc(100.0, 1.0, 0.1))  # 1.0000000000000056e-100