fix: abort PM enforcement if the script runs in a dependency

[neolitec]

The reason why is that NPM runs the preinstall script from the cache instead of from node_modules.

One way to detect that is by testing the presence of the _cacache which is the folder NPM uses for its cache.

Closes #13

[neolitec]

You can test it by creating a repository using NPM and try to install neolitec/repro-pnpm-project#fix.

https://github.com/neolitec/repro-pnpm-project/tree/fix

[robwil]

We are also seeing this problem on Mac, and this fix makes sense to me.

[Toilal]

Same issue here, on a github action pipeline with yarn v3.3.1. only-allow is executed when running "yarn install" on a project having a dependency containing only-allow preinstall script.

[antitoxic]

@zkochan is there any concerns regarding this? I would love to see this merged. Currently I have to work around this by having:

 "preinstall": "[[ $PWD == */node_modules/* ]] || npx only-allow pnpm"

instead of just npx only-allow pnpm but of course my workaround is OS-specific.