Make etc/ci/describe-system-config.sh POSIX-compliant (#1715) #695

Job	Run time
install (dev, 4.11.1, ubuntu-latest)	1h 2m 22s
install (dev, 4.11.1, macos-latest)	43m 49s
install (dev, 4.11.1, windows-latest, opam-repository-mingw: https://github.com/ocaml-opam/opam-r...	1h 40m 49s
install (8.18.0, 4.11.1, ubuntu-latest)	37m 37s
install (8.18.0, 4.11.1, macos-latest)	56m 54s
install (8.18.0, 4.11.1, windows-latest, opam-repository-mingw: https://github.com/ocaml-opam/opa...	2h 50m 3s
install (8.17.0, 4.11.1, ubuntu-latest)	42m 6s
install (8.17.0, 4.11.1, macos-latest)	1h 8m 4s
install (8.17.0, 4.11.1, windows-latest, opam-repository-mingw: https://github.com/ocaml-opam/opa...	2h 15m 31s
	11h 57m 15s

Provide feedback