mkcert is slow on darwin (due to nss support)

[crawshaw]

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

[crawshaw]

When actually generating a key, there are other sources of slowness. Total generation time with nss removed is 1s. About 100ms of that is spent in generateKey.

It seems to take mkcert 200ms to reach the beginning of the makeCert method.