provide prefix for mktemp

mktemp support is trivial.  However, the number of linux commands makes this process very linux dependent.

unsquashfs (no known cross-os tool)
http://zettelchen.blogspot.com/2009/04/build-squashfs-tools-for-mac-os-x.html

( could easily be done with another command, I would think)
brew tap iveney/mocha && brew install realpath

After installing these tools, rkt appeared to work. OS X Yosemite

```
$ rkt fetch https://github.com/coreos/etcd/releases/download/v0.5.0-alpha.4/etcd-v0.5.0-alpha.4-linux-amd64.aci
sha256-701c24b2d275f0e291b807a464ae2390bcd8d7c5b4f2d7e47e6fd917cd5e5588
```
Could you please use `$()` command substitution?