Move vendor dir in Python search path just before site-packages.

[dom96]

We have two functions that adjust the sys path in Python Workers. The first one (adjustSysPath) only runs when not restoring from a snapshot, so it is effectively immortalised in the memory snapshots. To avoid needing to regenerate all the memory snapshots (which for package snapshots would be a problem) we just rearrange the path at the same time as when we add the vendor path to the sys path.

Test Plan

$ bazel run @workerd//src/workerd/server/tests/python:vendor_dir_development@

Python sys.path: ['/session', '/lib/python312.zip', '/lib/python3.12', '/lib/python3.12/lib-dynload', '/session/metadata', '/session/metadata/vendor', '/lib/python3.12/site-packages', '/session/lib/python3.12/site-packages']

[danlapid]

/windsurf-review

[bhatiarajesh]

/windsurf review

[windsurf-bot]

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[hoodmane]

Thanks!

[dom96]

@hoodmane Hmm, looks like adjustSysPath needs to be called before the memory snapshot is collected (because site-packages need to be in the path for the package imports to work). I just reverted to its original. I guess proper thing to do is to add the site-packages path before memory snapshot is collected, but not the vendor dir. Though I'm not sure there is much point in doing this only in 0.27.7+.