This re-applies the fix in 5857 to make credentials.json writes concurrency safe.
The original fix used `path.Dir` instead of `filepath.Dir` - which led to not placing the temp file in the same folder (and drive) as the renamed file target. This led to errors on Windows environments where the working directory was on a different drive than the `~/.pulumi` directory. The change to use `filepath.Dir` instead ensures that even on Windows, the true directory containing the credentials file is used for the temp file as well.
Fixes#3877.
Running `pulumi` operations in parallel could occasionally result in truncating the `~/.pulumi/credentials.json` file and reading that truncated file from another process before the content could be written.
Instead, use `os.Rename` to atomically replace the file contents.
Concurrent `pulumi` operations could still compete for who gets to write the file first, and could lead to surprising results in some extreme cases. But we should not see the corrupted file contents any longer.
Fixes#3877.
