slugbuilder: Set STACK env var

This informs the official buildpacks that they should 
use binaries linked against newer versions of packages.

Closes #86

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Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>
util/cedarish: Track upstream changes

heroku/stack-images@55328da0ba8d4824ffbe779d82d6332714d11411
heroku/stack-images@7a5900da8f8860918a0a94c151b2adce62b1e406

[ci skip]

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>