Permission denied while reading dnsmasq.hosts

**Describe the bug**
When Multipass is started, it changes my external directory /mnt/Storage/multipass to mode 0700. This produces a "permission denied" error message for dnsmasq:

dnsmasq[2841]: cannot read /mnt/Storage/multipass/data/network/dnsmasq.hosts: Permission denied

**To Reproduce**
How, and what happened?
1. configure multipass to use an external storage with mode 0755
2. restart multipass
3. external storage directory will now be mode 0700

**Expected behavior**
multipass should not change external directory mode

**Logs**

[multipass.log](https://github.com/user-attachments/files/20393383/multipass.log)

**Additional info**
- OS: NAME="Ubuntu"
VERSION_ID="25.04"
VERSION="25.04 (Plucky Puffin)"
- 13th Gen Intel Core i5-13400
- multipass version: 1.15.1
- multipass local.driver : qemu
- desktop: Budgie:GNOME version: 10.9.2

**Additional context**
none

Reproduce the issue using the listed Ubuntu 25.04 steps with the QEMU driver, then trace the Multipass startup path that changes /mnt/Storage/multipass to mode 0700. Done means an externally configured directory retains its existing mode and dnsmasq can read data/network/dnsmasq.hosts without a permission-denied error.

Written by the indexing model from the issue text.