simplify string flag parsing #624

Job	Run time
homebrew-macos-test	5m 23s
distro-test (ubuntu:latest)	5m 11s
fresh-macos-test	5m 17s
distro-test (fedora:latest)	5m 10s
login-shells-and-linting	1m 59s
distro-test (debian:latest)	5m 11s
distro-test (alpine:latest)	5m 10s
distro-test (manjarolinux/base)	5m 11s
distro-test (opensuse/leap)	5m 11s
distro-test (opensuse/tumbleweed)	5m 11s
distro-test (kalilinux/kali-rolling)	5m 12s
	54m 6s

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