[Bug] pulsar-admin functions update does not update the function binary from a local file

### Search before asking

- [X] I searched in the [issues](https://github.com/apache/pulsar/issues) and found nothing similar.

### Version

Similar to #18966 and #18972, pulsar-admin seems unable to correctly update a running function from a local file, despite seeming to succeed. In fact, it seems as if it is impossible to update running pulsar function code in any way whatsoever.

```
$ go build -o funcbin
$ sha256sum funcbin
bfb2284411e0bdb893a91d93addb88b9e61c4b3d04f17dae94b57db6a1e1dc2d funcbin
$ pulsar-admin functions download --fqfn public/default/running_function --destination-file `pwd`/current_func
Downloaded successfully
$ sha256sum current_func
074f137112ff7d7050c6a10cabcb94276f3066b87ca55ebc7e6699e7e92ad02b current_func
$ pulsar-admin functions update --go `pwd`/funcbin --fqfn public/default/running_function
Updated successfully
$pulsar-admin functions download --fqfn public/default/running_function --destination-file `pwd`/updated_func
Downloaded successfully
sha256sum updated_func
074f137112ff7d7050c6a10cabcb94276f3066b87ca55ebc7e6699e7e92ad02b updated_func
```

### Minimal reproduce step

(see above)

### What did you expect to see?

Function is actually updated.

### What did you see instead?

Update appears to succeed. Nothing is actually done.

### Anything else?

_No response_

### Are you willing to submit a PR?

- [ ] I'm willing to submit a PR!

Reproduce the issue with the listed `pulsar-admin functions update` and `functions download` commands using the local `funcbin`, then compare the SHA-256 hashes. Trace the functions update and download entry points to determine why the downloaded binary remains unchanged. Done means a local-file update is reflected in the subsequently downloaded function binary.

Written by the indexing model from the issue text.