Starting from Big Sur 11.3 certificate installation requires user interaction

First, thank you so much for mkcert! I love it and use it often ⭐

It seems that some default behavior may have changed in macOS 11, as mkcert isn't working out of the box on GitHub Actions with `macos-latest`. (Ah yes, I just read an issue I referenced and it said `Starting from Big Sur 11.3 certificate installation requires user interaction, however it can be bypassed by using SecItemAdd method.`)

I'm getting the same error as reported in https://github.com/FiloSottile/mkcert/issues/94

And [I was advised that](https://github.com/actions/virtual-environments/issues/4519) I could compile some swift code to solve my issue, similar to what they did in this PR https://github.com/actions/virtual-environments/pull/3311.

```
swiftc add-certificate.swift
sudo ./add-certificate YourCertName.cer
```

By the time I was done, however, I wasn't using `mkcert` anymore. I thought this would be a more appropriate solution to build into mkcert.