Fix Bootsnap version hash override

[rafaelfranca]

Summary

• Fix the Bootsnap version hash cache to use the writer-compatible instance variable.
• Add regression coverage for an override after the first read.

Verification

• Focused Bootsnap regression fixture passed.
• Bootsnap cache smoke test passed.
• Ruby syntax checks passed.
• git diff --check passed.

[palkan]

Thanks!