fix: add -Iperl5 so TrueNASMultipath.pm can find its parent in CI lint

TrueNASMultipath.pm inherits from TrueNAS.pm which lives in perl5/. The
-I/tmp/pve-stub flag alone doesn't make that path searchable, so the
'use base' failed at compile time. Adding -Iperl5 fixes it.

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

.github/workflows/build.yml

@@ -46,7 +46,7 @@ jobs:
           echo "==> Checking Perl syntax..."
           perl -c -I/tmp/pve-stub perl5/PVE/Storage/Custom/TrueNAS.pm \
             && echo "TrueNAS.pm: OK"
-          perl -c -I/tmp/pve-stub -I/tmp/pve-stub/.. \
+          perl -c -I/tmp/pve-stub -Iperl5 \
             perl5/PVE/Storage/Custom/TrueNASMultipath.pm \
             && echo "TrueNASMultipath.pm: OK"