CI: check each individual file in a new shellcheck instance

shellcheck runs very slow when operating on many files at once and has the a high potential to hang on some systems when doing so.

.github/workflows/shellcheck.yml

@@ -59,6 +59,8 @@ jobs:
         run: |
           cd $GITHUB_WORKSPACE
           sudo apt install shellcheck wget -y
-          if ! shellcheck api createapp gui install manage preload settings uninstall updater etc/preload-daemon etc/runonce-entries etc/terminal-run apps/*/install apps/*/install-32 apps/*/install-64 apps/*/uninstall --color=always | sed '0,/'$(echo -e "\033")'\[31m/!{q1}' ;then
-            exit 1
-          fi
+          for file in api createapp gui install manage preload settings uninstall updater etc/preload-daemon etc/runonce-entries etc/terminal-run apps/*/install apps/*/install-32 apps/*/install-64 apps/*/uninstall; do
+            if ! shellcheck "$file" --color=always | sed '0,/'$(echo -e "\033")'\[31m/!{q1}' ; then
+              exit 1
+            fi
+          done