Adding linting for initcpio scripts
archiso/initcpio/install/*: Setting bash shebang for all scripts and making them comform with shellcheck.
archiso/initcpio/{hooks,script}/*: Setting ash shebang for all scripts and making them comform with shellcheck (for dash, as shellcheck has no ash specific ruleset). Essentially the ash based scripts should be POSIX compliant as much as possible to have an easier time writing, debugging and maintaining them.
.gitlab-ci.yml: Integrating shellcheck for initcpio scripts.
Closes #32 (closed)