Only materialize immutable files once per process

[thejcannon]

This PR removes TempImmutableLargeFile in favor of having the caller provide a writer function which writes to the tokio::fs::File, and we wrap the writing in the necessary setup/teardown code.

This has a few benefits:

• We can use a OnceCell to ensure the file is only trying to materialized by one thread
• We can cleanup on error now (whoops that was embarrassing)
• Eliminates a heisenbug I was seeing where we'd fail to hardlink with "No such file or directory".
  • I still don't understand the heisenbug, and I'm still worried we'd see it cross-process 😢

All in all, big win