Agda not using caches for primitive modules

[laMudri]

As discussed at agda/agda#4526, Agda 2.6.1 is performing very badly relative to Agda 2.6.0.1. It seems that the issue is in loading primitive modules, which is taking ~20 seconds on my machine. On the issue I linked to before, we concluded that .agdai files weren't being generated for the primitives, and that Agda was just checking them again every time. However, looking in the relevant /nix/store/ path, this doesn't seem to be true, and the script introduced in #226 is still working to an extent. Probably, though, we need to do something to that script to fix this issue.

/cc @abbradar

[laMudri]

To be resolved upstream. agda/agda#4526