tools: update paths in v8_external_snapshot.gypi

Update the paths to source files in v8_external_snapshot.gypi this was broken and went unnoticed since this target is never really built by default.

Fixes: https://github.com/nodejs/node/issues/28964

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

/cc @refack @gengjiawen

I'm not sure how relevant this is and if CI can even check for this. Maybe we should consider just removing this target at some point?