Add shellcheck to ci.installer.yml #122

Job	Run time
eval-posix-sh	17s
eval-zsh	22s
install-pkgx (macos-latest)	6s
install-pkgx (ubuntu-latest)	11s
eval-bash	13s
linuxen (debian:buster-slim)	7s
linuxen (debian:bullseye-slim)	7s
multiple-apply-is-ok	16s
linuxen (debian:bookworm-slim)	8s
no-dirs (/usr/local)	14s
linuxen (archlinux:latest)	28s
shellcheck	3s
no-dirs (/usr/local/bin)	12s
sudo-not-required	8s
linuxen (ubuntu:focal)	7s
linuxen (ubuntu:jammy)	7s
sudo-required	13s
upgrades	16s
linuxen (ubuntu:24.04)	19s
usage-as-proxy	19s
linuxen (ubuntu:24.10)	13s
linuxen (fedora:latest)	22s
	4m 48s

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