mkcert is slow on darwin (due to nss support)

On a MacBook Air 2018:

```
$ time (./mkcert -help &> /dev/null)

real	0m1.010s
user	0m0.657s
sys	0m0.309s
```

This is caused by the `init` function in truststore_nss.go, which invokes the brew command. If I comment out the init function:

```
$ time (./mkcert -help &> /dev/null)

real	0m0.025s
user	0m0.013s
sys	0m0.010s
```