ci: enable shellcheck -x to follow sourced modules

Passes -x (--external-sources) to shellcheck so it follows the
'# shellcheck source=...' directives in build.sh and checks the
split modules in their sourced context. Without this, every sourced
module triggers SC1091 (can't follow source) plus SC2154/SC2034
noise from cross-file variable usage.

Also quotes $script_dir inside $(dirname $script_dir) in
scripts/packaging/rpm.sh — the heredoc-embedded command
substitution tripped SC2086 once shellcheck started analyzing the
subshell context.

Co-Authored-By: Claude <claude@anthropic.com>

.github/workflows/shellcheck.yml

@@ -29,4 +29,4 @@ jobs:
           sudo apt update && sudo apt install -y shellcheck
       - name: shellcheck
         run: |
-          git grep -l '^#\( *shellcheck \|!\(/bin/\|/usr/bin/env \)\(sh\|bash\|dash\|ksh\)\)' -- '*.sh' | xargs shellcheck
+          git grep -l '^#\( *shellcheck \|!\(/bin/\|/usr/bin/env \)\(sh\|bash\|dash\|ksh\)\)' -- '*.sh' | xargs shellcheck -x