Forge build fails on macos-latest with "Loading configuration [FAILED: Cannot use 'import.meta' outside a module]"

### Pre-flight checklist

- [x] I have read the [contribution documentation](https://github.com/electron/forge/blob/main/CONTRIBUTING.md) for this project.
- [x] I agree to follow the [code of conduct](https://github.com/electron/electron/blob/main/CODE_OF_CONDUCT.md) that this project uses.
- [x] I have searched the issue tracker for a bug that matches the one I want to file, without success.

### Forge version

7.9.0

### Electron version

38.2.2

### Operating system

`macos-latest` (GitHub Actions)

### Last known working Forge version

_No response_

### Expected behavior

Build successfully

### Actual behavior

Build fails
```
> electron-forge make

❯ Checking your system
❯ Checking package manager version
✔ Found npm@10.9.3
✔ Checking your system
25h❯ Loading configuration
Failed to load: /Users/runner/work/structure/structure/client/forge.config.mts
✖ Loading configuration [FAILED: Cannot use 'import.meta' outside a module]

An unhandled rejection has occurred inside Forge:
SyntaxError: Cannot use 'import.meta' outside a module
25h
Error: Process completed with exit code 1.
```
(the `package.json` *does* have `"type": "module",`)

### Steps to reproduce

Happens only on GitHub Actions `macos-latest`
Sample repo: https://github.com/neopostmodern/structure (`client` subpackage, or run `npm run client:build:electron` from root package)

### Additional information

Seems similar to #4029, but changing the node.js version has no effect
It was a non-ESM project before, then the error showed up and I hoped switching to ESM entirely would fix it (but it didn't)