`mixin` ignores `when` branching

[ggibson]

Is this supposed to work? I couldn't find an issue about it. No matter the 
`when` branch, the `mixin` is always used.
    
    
    #tokens.nim
    proc getToken*(): string = "123456"
    
    #main.nim
    import tokens
    template login(): bool =
      when not defined(release):
        mixin getToken
      getToken() != "INVALID"
    block testBad:
      proc getToken(): string = "INVALID"
      assert login() == false
    
    
    Run