Add check target to Makefile

Makefile:

• Move shellcheck commands from .gitlab-ci.yml.

.gitlab-ci.yml:

• Run make check.
• Prevent partial upgrades, i.e. use -Syu.
• Prevent reinstalling already installed packages by using pacman's --needed option.
• Don't force-download the package database.

.github/workflows/lint.yml:

• GitHub Actions workflow that runs make check.