Hello, does anyone know of a tool to characterize a standard cell?

You would have to use a spice simulation software to simulate the spice files accompanying those standard cells in the PDK.

but there are no tools to characterize wings and generate the .LIB?

I don't know of any open source or free tool that can do this. Commercially, there is Cadence Liberate and Synopsys PrimeLib. Spice simulations can generate the required information (slews, delays), but in order to convert this information into Liberty format, it needs to be swept across a range of input slews and output capacitances and then dumped into the Liberty format.

HiTas/Yagle (https://gitlab.lip6.fr/vlsi-eda/tas-yagle) should be a tool that should allow to make liberty files. Unfortunately current Sky130 models are not compatible with the tool.