How can I force Steam to run a post installation script again?

I don't think so, maybe go to the game's properties (on Steam) and go to the local files tab and click on the verify Game files. It'll basically see if any files are missing and u can choose if you would like to reinstall them.

From https://partner.steamgames.com/doc/sdk/installscripts :

The install script is marked in the depot manifest. You can see this in the generated manifest.txt file as a 100 in the Flags field.

When a Steam user is starting a game, Steam will scan all of the mounted depots for that game for any file with the install script flag and run them.

From what I understand, you need to find that manifest.txt file for specific game and update this Flags field back to initial value (100).

This answer suggests that for each dependency Steam may actually create a registry entry, which means it may be required to clean the registry before trying to relaunch install scripts.

And it also works only for Windows:

NOTE: Install script functionality described below is primarily for Windows operating systems. MacOS support is limited to file permissions and symlinks. There is no Linux/SteamOS install script functionality at this time.

Unfortunately, I don't have Windows machine right now and cannot validate this.