fix: remove double slash in bundled extension paths

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Built bundles:

• Postgres WASM: https://github.com/electric-sql/pglite/actions/runs/10833829583/artifacts/1925511235
• Interim build files: https://github.com/electric-sql/pglite/actions/runs/10833829583/artifacts/1925511407
• PGlite: https://github.com/electric-sql/pglite/actions/runs/10833829583/artifacts/1925542265
• Demos: https://github.com/electric-sql/pglite/actions/runs/10833829583/artifacts/1925558809

[github-actions[bot]]

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