fix: set shellcheck severity threshold to warning in CI

SC1091 (cannot follow sourced paths), SC2012 (ls in glob for kernel
package filenames), and SC2016 (literal $ in Makefile heredoc) are
info-level false positives that do not represent real bugs. Use -S warning
so only genuine warnings and errors fail the build.

Co-authored-by: Ona <no-reply@ona.com>

Author: Interested-Deving-1896

.github/workflows/build.yml

@@ -31,7 +31,7 @@ jobs:
       - name: Lint
         run: |
           find . -name '*.sh' ! -path './.cache/*' ! -path './kernel/src/*' \
-            -exec shellcheck -x {} +
+            -exec shellcheck -x -S warning {} +
 
   build-matrix:
     name: Build (${{ matrix.distro }} / ${{ matrix.mode }})