Incorrect path OS detection

## Environment

* Operating system (including version): Ubuntu 20.04
* mkcert version (from `mkcert -version`): v1.4.4
* Server (where the certificate is loaded):
* Client (e.g. browser, CLI tool, or script):

## What you did

`mkcert -install`

## What went wrong

> ERROR: failed to execute "update-ca-trust extract": exit code 1
> sudo: update-ca-trust: command not found

When calling the `mkcert -install`, I get an error because I have a directory specific to **CentOS/RedHat**. But, I have a different OS and there is a specific path for it.

https://github.com/FiloSottile/mkcert/blob/2a46726cebac0ff4e1f133d90b4e4c42f1edf44a/truststore_linux.go#L36

I think that it is necessary to change the definition method to get rid of this problem.