Installing project using bolt on Windows fails with "$binary is not a symlink"

| Title | Description
| --------------------|------------
| Version | [0.21.2](https://ci.appveyor.com/project/electron-userland/electron-forge/builds/22126111#L215), [0.22.6](https://ci.appveyor.com/project/electron-userland/electron-forge/builds/22126537#L215), [master](https://travis-ci.org/boltpkg/bolt/jobs/488853236#L488)
| Type | Issue
| node | 6
| Operating System | Windows
| Short Description | `bolt` fails with "$binary is not a symlink"

It seems that the binary symlinking code doesn't work on Windows. The exact version numbers link to AppVeyor builds for Electron Forge where running `bolt` fails when it tries to install `acorn`'s `acorn.ps1` file. `master` links to a testsuite run of `bolt` itself (from #217), specifically one of the symlink-related tests that fail.