CI: shellcheck: remove apk upgrade command (MR 2511)

CI is currently failing at the "apk upgrade" line with the following
error. It must be related to the openssl3 transition that is going on in
Alpine:

485B6261A57F0000:error:0A000086:SSL routines:tls_post_process_server_certificate:certificate verify failed:ssl/statem/statem_clnt.c:1882:
ERROR: musl-1.2.2-r6: Permission denied
485B6261A57F0000:error:0A000086:SSL routines:tls_post_process_server_certificate:certificate verify failed:ssl/statem/statem_clnt.c:1882:
ERROR: busybox-1.34.0-r3: Permission denied
485B6261A57F0000:error:0A000086:SSL routines:tls_post_process_server_certificate:certificate verify failed:ssl/statem/statem_clnt.c:1882:
ERROR: musl-utils-1.2.2-r6: Permission denied

So let's get rid of it. It shouldn't be needed to install shellcheck
anyway.

.gitlab-ci.yml

@@ -57,7 +57,6 @@ shellcheck:
   <<: *only-default
   image: alpine:edge
   before_script:
-    - apk -q upgrade
     - apk -q add shellcheck
   script:
     - .gitlab-ci/shellcheck.sh