Fix invalid fallback in bench-run.sh

[fallintoplace]

Summary

Fix the fallback branch in scripts/bench-run.sh so unknown package managers report a normal error instead of crashing with a bad shell substitution.

Root cause

The default case branch used ${$1}, which triggers bad substitution in bash before the intended message can be printed. It also used return 1 even though this branch runs at script top level.

Fix

• use ${PACKAGE_MANAGER} in the error message
• use exit 1 for the script-level failure path

Verification

• bash -n scripts/bench-run.sh
• bash scripts/bench-run.sh bogus gatsby /tmp/berry-bench-check now prints Invalid package manager bogus and exits non-zero