fix: scan all dirs when script_dirs has multiple entries

[fedemaleh]

Summary

The reusable shellcheck workflow documents script_dirs as "Space-separated dirs/files to scan", but the implementation runs find "$SCRIPT_DIRS" with the variable quoted — so a multi-dir value like entrypoint scripts is treated as a single path (entrypoint scripts), which doesn't exist. find then errors and shellcheck runs with no files:

find: 'entrypoint scripts': No such file or directory
No files specified.
##[error]Process completed with exit code 123.

Any caller passing more than one dir/file hits this. A single-dir value works, which is why it went unnoticed.

Fix

Split the space-separated value into an array and pass each element as a distinct find argument, so the documented contract works while keeping each path quoted:

read -ra dirs <<< "$SCRIPT_DIRS"
find "${dirs[@]}" -type f -print0 | xargs -0 shellcheck --severity="$SEVERITY"

No change to the empty-input branch or to the env:-based input handling (no injection surface added).

Test plan

• Caller with a single dir (script_dirs: scripts) — still works.
• Caller with multiple dirs (script_dirs: entrypoint scripts) — now scans both (previously errored with exit 123).
• Empty script_dirs — unchanged (recursive *.sh).

Context

Found while adding PR checks to nullplatform/application-lifecycle-manager, which passes script_dirs: entrypoint scripts. That repo currently works around this by calling the reusable workflow once per directory; this fix lets callers pass the space-separated list as documented.