homebrew client has "/usr/local" incorrectly hardcoded, login fails

I tried to run "keybase login" after a "brew install" with my homebrew installed at /homebrew:

``` sh
~ (osx) % keybase login
▶ INFO Checking service
▶ INFO Service: Not Installed
▶ INFO Installing Keybase service
Unloading keybase.service
Unloading homebrew.mxcl.keybase
▶ ERROR Error installing Keybase service: stat /usr/local/opt/keybase/bin/keybase: no such file or directory
▶ ERROR stat /usr/local/opt/keybase/bin/keybase: no such file or directory
~ (osx) % brew --prefix
/homebrew
~ (osx) % which keybase
/homebrew/bin/keybase

```

Please consider using `$(brew --prefix)` instead of `/usr/local`.

Start from the `keybase login` service-installation path and locate where `/usr/local/opt/keybase/bin/keybase` is constructed. Reproduce with `brew --prefix` set to `/homebrew`, then verify that login uses the Homebrew prefix and no longer reports the missing path.

Written by the indexing model from the issue text.