Only set arch on bin dir when we are building for Windows

[RebeccaMahany]

We are currently setting the bin dir to /usr/local/kolide-k2/bin/amd64 on Linux, which we don't want to do yet. This PR makes sure we only add the arch to the bin dir if we're building for Windows.

[James-Pickett]

nice!

[RebeccaMahany]

Tested for macOS:

launcher-1.10.1.flat.pkg % tree Payload/usr 
Payload/usr
└── local
    └── kolide-k2
        ├── Kolide.app
        │   └── Contents
        │       ├── CodeResources
        │       ├── Info.plist
        │       ├── MacOS
        │       │   └── launcher
        │       ├── Resources
        │       │   └── Kolide.icns
        │       ├── _CodeSignature
        │       │   └── CodeResources
        │       └── embedded.provisionprofile
        ├── bin
        │   ├── launcher -> ../Kolide.app/Contents/MacOS/launcher
        │   └── osqueryd -> ../osquery.app/Contents/MacOS/osqueryd
        └── osquery.app
            └── Contents
                ├── CodeResources
                ├── Info.plist
                ├── MacOS
                │   └── osqueryd
                ├── PkgInfo
                ├── Resources
                │   └── osqueryctl
                ├── _CodeSignature
                │   └── CodeResources
                └── embedded.provisionprofile

14 directories, 15 files