[Taildrive] User server stopped with error "System Administrator" is root

### What is the issue?

I'm trying to mount a shared directory on my Linux computer using `davfs2`. It mounts successfully, but the shared folder contents are empty.

```
# tree /mnt/tailscale/user@domain.com/
/mnt/tailscale/user@domain.com/
├── minipc
├── mobile
└── pi
└── obsidian

5 directories, 0 files
```

Checking the `tailscaled` logs on the server show the following errors:

```
ago 27 19:41:35 server tailscaled[923]: user server /nix/store/b4g0y694sx9ksgcmfv3ykblwx5by7h9v-tailscale-1.72.0/bin/.tailscaled-wrapped stopped with error "System administrator" is root, will try again in 16.384s
ago 27 19:41:51 server tailscaled[923]: user server /nix/store/b4g0y694sx9ksgcmfv3ykblwx5by7h9v-tailscale-1.72.0/bin/.tailscaled-wrapped stopped with error "System administrator" is root, will try again in 30s
ago 27 19:42:21 server tailscaled[923]: user server /nix/store/b4g0y694sx9ksgcmfv3ykblwx5by7h9v-tailscale-1.72.0/bin/.tailscaled-wrapped stopped with error "System administrator" is root, will try again in 2ms
ago 27 19:42:21 server tailscaled[923]: user server /nix/store/b4g0y694sx9ksgcmfv3ykblwx5by7h9v-tailscale-1.72.0/bin/.tailscaled-wrapped stopped with error "System administrator" is root, will try again in 4ms
ago 27 19:42:21 server tailscaled[923]: user server /nix/store/b4g0y694sx9ksgcmfv3ykblwx5by7h9v-tailscale-1.72.0/bin/.tailscaled-wrapped stopped with error "System administrator" is root, will try again in 8ms
ago 27 19:42:21 server tailscaled[923]: user server /nix/store/b4g0y694sx9ksgcmfv3ykblwx5by7h9v-tailscale-1.72.0/bin/.tailscaled-wrapped stopped with error "System administrator" is root, will try again in 16ms
ago 27 19:42:21 server tailscaled[923]: user server /nix/store/b4g0y694sx9ksgcmfv3ykblwx5by7h9v-tailscale-1.72.0/bin/.tailscaled-wrapped stopped with error "System administrator" is root, will try again in 32ms
ago 27 19:42:21 server tailscaled[923]: [RATELIMIT] format("user server % v stopped with error %v, will try again in %v")
ago 27 19:42:38 server tailscaled[923]: [RATELIMIT] format("user server % v stopped with error %v, will try again in %v") (8 dropped)

```

The logs can be tracked to: https://github.com/tailscale/tailscale/blob/31cdbd68b12790dfa6c432844a8340563286bc58/drive/driveimpl/remote_impl.go#L321

### Steps to reproduce

1. Apply relevant ACL settings

```json
{
"nodeAttrs": [
{
"target": ["*"],
"attr": ["drive:access", "drive:share"]
}
],
"grants": [
{
"src": ["autogroup:member"],
"dst": ["autogroup:self"],
"app": {
"tailscale.com/cap/drive": [
{
"shares": ["*"],
"access": "rw"
}
]
}
}
]
}
```

2. Share directory as `root`

```bash
server# tailscale drive share shared-folder /path/to/shared/folder
```

3. Mount shared directory on another machine using `davfs2`:

```bash
desktop# mount -t davfs http://100.100.100.100:8080 /path/to/mountpoint
```

4. Review the contents of the mountpoint.

### Are there any recent changes that introduced the issue?

_No response_

### OS

Linux

### OS version

`Linux desktop 6.6.45 #1-NixOS SMP PREEMPT_DYNAMIC Sun Aug 11 10:47:28 UTC 2024 x86_64 GNU/Linux`

### Tailscale version

1.70.0

### Other software

_No response_

### Bug report

BUG-a6e3efb5caca7312f773e990d68527d5870e9fb28fc1b009369703b5ad445ec8-20240827223802Z-61ef714733fce97c