Add shellcheck to ci.installer.yml

[abitrolly]

No description provided.

[jhheider]

Always prefer a good lint.

[abitrolly]

The action doesn't seem to work. At least this error should be captured.

$ pkgx shellcheck tmp/installer.sh -S style
...
In tmp/installer.sh line 29:
      $SUDO apt-get $cmd -qq -o=Dpkg::Use-Pty=0 $@
                    ^--^ SC2086 (info): Double quote to prevent globbing and word splitting.
                                                ^-- SC2068 (error): Double quote array expansions to avoid re-splitting elements.

[abitrolly]

reviewdog only reports changed lines - reviewdog/action-shellcheck#72 - but necessary to check settings to ensure it reports warning info's etc.