electron-forge makers does not support arch

[liudonghua123]

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• I have searched the issue tracker for an issue that matches the one I want to file, without success.

Please describe your issue:

I tried to build https://github.com/felixrieseberg/windows95, and the content of forge.config.js#L30 is "setupExe: windows95-${package.version}-setup-${process.arch}.exe", And I run electron-forge make --arch=ia32,x64 on my Windows 10 64bit. the files generated are

.
├── ia32
│   ├── RELEASES
│   ├── windows98-1.3.0-full.nupkg
│   └── windows98-win32-1.3.0-setup-x64.exe
└── x64
    ├── RELEASES
    ├── windows98-1.3.0-full.nupkg
    └── windows98-win32-1.3.0-setup-x64.exe

The install file is the same, When I delete this line of config, the files generated are

.
├── ia32
│   ├── RELEASES
│   ├── windows98-1.3.0-full.nupkg
│   └── windows98-1.3.0 Setup.exe
└── x64
    ├── RELEASES
    ├── windows98-1.3.0-full.nupkg
    └── windows98-1.3.0 Setup.exe

It was the same too.

But in other makers like deb, rpm, zip, different arch generate different files like

├── windows98_1.3.0_amd64.deb
├── windows98_1.3.0_i386.deb
├── windows98-1.3.0.i386.rpm
├── windows98-1.3.0.x86_64.rpm
├── windows98-linux-ia32-1.3.0.zip
└── windows98-linux-x64-1.3.0.zip

So I have two ideas about this.

1. change https://github.com/electron-userland/electron-forge/blob/master/packages/maker/squirrel/src/MakerSquirrel.ts#L33 include targetArch like https://github.com/electron-userland/electron-forge/blob/master/packages/maker/deb/src/MakerDeb.ts#L35 does.
2. provide the correct targetArch envrionment in makers configuration.

I want to deploy the generate files to github release via travis-ci, so the same filename file will overwrite the exists one.

[liudonghua123]

I write a #611 to fix this problem.

[malept]

You can specify arch by setting the maker's config to be a function. For example: electron/fiddle#125