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.
Ensuring that variables are not treated as options and introducing variable quoting.

.gitlab-ci.yml:
Integrating shellcheck for initcpio scripts.

Closes #32