`mkcert -csr` cannot be mixed with `-client` — most likely a bug due to presence of checks for `-client` in code

## Environment

* Operating system (including version): `NixOS 25.05.20250418.2a76610 (Warbler)`
* mkcert version (from `mkcert -version`): `v1.4.4`
* Server (where the certificate is loaded): n/a
* Client (e.g. browser, CLI tool, or script): n/a

## What you did

`mkcert -client -csr ./vsh-csr.pem -cert-file ./vsh.pem`

## What went wrong

```
ERROR: can only combine -csr with -install and -cert-file
```

## Additional info
- `main()` [checks that `-client` and `-csr` are not passed together](https://github.com/FiloSottile/mkcert/blob/1c1dc4ed27ed5936046b6398d39cab4d657a2d8e/main.go#L139-141)
- `makeCertFromCSR()` [checks `client` flag](https://github.com/FiloSottile/mkcert/blob/1c1dc4ed27ed5936046b6398d39cab4d657a2d8e/cert.go#L247), which is always false because of the check above
- **Suggestion**: relax check in `main()` to allow passing `-client` to `mkcert -csr` invocations and enable this code path