Unable to launch locally built multipass, without installing it first.

**Describe the bug**
After following the steps in [BUILD.linux.md](https://github.com/canonical/multipass/blob/main/BUILD.linux.md), I was unable to successfully launch `multipass`, and was getting the following error:
```sh
[error] [client] Caught an unhandled exception: failed to open file '/usr/local/etc/multipassd/multipass_root_cert.pem': No such file or directory(2)
```
Note that after installing the official snap of `multipass`, the error was not occurring again (even after uninstalling it).

**To Reproduce**
How, and what happened?
1. Either use a whole new Ubuntu, or use `multipass` to start a fresh Ubuntu machine, with enough RAM and disk space to be able to build the project (--memory=8gb --disk=40gb did it for me).
2. Clone the `multipass` repository on the machine.
3. Follow the steps inside [BUILD.linux.md](https://github.com/canonical/multipass/blob/main/BUILD.linux.md).
Note 1: I ended up with 3 build errors, that I either fixed by adding = 0 after uninitialized variables, or discarded by commenting the `gtk_window_set_icon` call.
Note 2: I explicitly put --parallel 2 or 1, in order for the process to not get killed because of OOM issues.
4. Start the daemon with: `sudo ./build/bin/multipassd`.
5. Then start the client with: `./build/bin/multipass`.
=> You should get the error mentioned above.

**Expected behavior**
If properly following the indicated steps, Multipass should be able to launch without any issue.

**Logs**
Unable to provide the logs at the moment, I've uninstalled `multipass` from my host again.

**Additional info**
- OS: Ubuntu 26.04
- CPU architecture or model: x64
- `https://github.com/canonical/multipass/commit/c74afa86b062ed64439afbadc4dc45fefe754c5b`

Start with BUILD.linux.md and reproduce the documented build and launch sequence using ./build/bin/multipassd and ./build/bin/multipass. Trace how the locally built client expects /usr/local/etc/multipassd/multipass_root_cert.pem to be provided, then verify the fix by launching Multipass without installing the official snap.

Written by the indexing model from the issue text.