fix: use SCRIPTDIR so shellcheck can find COMMON on CI

SHELLCHECK_OPTS used $GITHUB_WORKSPACE which GitHub Actions does not
expand in env: blocks. SCRIPTDIR is shellcheck's built-in token that
resolves to the directory of the script being checked, so shellcheck
can find COMMON alongside each test script without a hardcoded path.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: andy5995

.github/workflows/shellcheck.yml

@@ -21,4 +21,4 @@ jobs:
       - name: Run ShellCheck
         uses: ludeeus/action-shellcheck@master
         env:
-          SHELLCHECK_OPTS: -x -P $GITHUB_WORKSPACE/test
+          SHELLCHECK_OPTS: -x -P SCRIPTDIR

test/test_basic.sh

@@ -125,4 +125,3 @@ cmp_substr "$output" "Skipping"
 
 echo "Basic tests passed"
 exit 0
-