Fix macOS desktop build failure in CI

The GitHub-hosted runners began experiencing issues while building macOS
desktop distributions, exclusively affecting the macOS environment.
The Ubuntu and Windows environments remained unaffected.

The logs highlighted the absence of Python in the macOS environment, which
resulted in build failure:

```sh
  Error: Exit code: ENOENT. spawn /usr/bin/python ENOENT
```

Since the `electron-builder` package uses Python scripts to create DMG
disk images for macOS distributions, Python is needed for building the
application. However, electron-builder uses Python 2.X meanwhile modern
macOS versions have removed Python 2.X from the operating system on
default installation.

Although this issue was resolved in `electron-builder` version 23,,
`vue-cli-plugin-electron-builder` continues to use version 22. Due to a
lack of maintenance, the package is unlikely to receive updates.

This commit forces `vue-cli-plugin-electron-builder` to use the latest
`electron-builder` which resolves the macOS distribution build failure.

In CI process, GitHub-hosted runners start to fail when building macOS
desktop distributions. It is only observered in the macOS environment
while the application is built successfully in both the Ubuntu and
Windows environments.

The error message in the logs indicated that Python was not found in the
macOS environment:

```sh
Error: Exit code: ENOENT. spawn /usr/bin/python ENOENT
```

`electron-builder` package uses Python scripts for certain operations,
specifically for creating DMG disk images for macOS distributions. As a
result, Python is a necessary dependency when building the application
for macOS.

`electron-builder` has fixed this starting from version 23, but
vue-cli-plugin-electron-builder still refers to version 22 and it is
unmaintained and not likely to get updates.

The solution is to add a step in the GitHub Actions workflow to set up
Python in the macOS environment. `actions/setup-python` sets up the
Python environment in the runner if the OS is macOS.

This change does not impact the Ubuntu and Windows environments as the
setup-python step is conditionally executed only for macOS. The addition
of Python to the macOS environment in CI process has resolved the build
failure issue for the macOS distribution.

See also:

- electron-userland/electron-builder#6606
- electron-userland/electron-builder#6726
- electron-userland/electron-builder#6732
- nklayman/vue-cli-plugin-electron-builder#1691
- nklayman/vue-cli-plugin-electron-builder#1701