fix: on linux fall back to native arch if npm_config_arch is not resolved

[MarshallOfSound]

As of https://github.com/desktop/dugite/pull/392 anyone running dugite on linux with npm_config_arch=ia32 set will be unable to use dugite where they previously could. This at least gets it working again by falling back to os.arch() if the npm_config_arch binary couldn't be located.

Technically fixes a breaking change

[niik]

@dennisameling would you mind taking a look at this when you have a moment since you authored #392? Thanks!

[dennisameling]

Sure, will have a look in a few hours from now!