simplify: use existing prefix as archivePrefix, fixes #937

[ikreymer]

Description

Fixes #937

Use existing prefix for wabac.js archivePrefix instead of recomputing, ensures passthrough handles mount prefix.

Update wabac.js to 2.22.15
bump to 2.9.0b1

Motivation and Context

Screenshots (if appropriate):

Types of changes

• Replay fix (fixes a replay specific issue)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added or updated tests to cover my changes.
• All new and existing tests passed.