frontend: ignore AccessDenied when writing builtin.zig

This issue already existed in master branch, however, the more
aggressive caching of builtin.zig in this branch made it happen more
often. I added doc comments to AtomicFile to explain when this problem
can occur.

For the compiler's use case, error.AccessDenied can be simply swallowed
because it means the destination file already exists and there is
nothing else to do besides proceed with the AtomicFile cleanup.

I never solved the mystery of why the log statements weren't printing
but those are temporary debugging instruments anyway, and I am already
too many yaks deep to whip out another razor.

closes #14978

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