migrate.rb: work on Linux, too

Asherah Connorrequested to merge
push-oqoqnysvqosk into master

What does this MR do and why?

migrate.rb always fails on Linux because command isn't a binary on it.

system("command", "-v", command, out: File::NULL) is actually executing the command binary on PATH (!). macOS provides one at /usr/bin/command which is just this shell script:

#!/bin/sh
# $FreeBSD$
# This file is in the public domain.
builtin `echo ${0##*/} | tr \[:upper:] \[:lower:]` ${1+"$@"}

Lol! Anyway. It's not there on Linux! Let's just call which instead, which is present on PATH on both macOS and Linux.

(The alternative would be to use the shell form of Kernel#system, but why involve a shell when we can avoid it.)

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