cmd/link: change `default_prefix?` check to `/usr/local` check
cmd/link: change `default_prefix?` check to `/usr/local` check

Linking macOS-provided software breaks things only in `/usr/local`
prefixes, hence the `default_prefix?` check, which was included when our
only default prefix on macOS was `/usr/local`. Now that we install into
`/opt/homebrew` too, the default prefix check is needlessly restrictive.

JCount/brew,nandub/brew,MikeMcQuaid/brew,Homebrew/brew,nandub/brew,EricFromCanada/brew,JCount/brew,MikeMcQuaid/brew,JCount/brew,MikeMcQuaid/brew,nandub/brew,EricFromCanada/brew,EricFromCanada/brew,nandub/brew,EricFromCanada/brew,JCount/brew,Homebrew/brew,Homebrew/brew,MikeMcQuaid/brew,Homebrew/brew