Make sure to set CF_STACK during staging on cflinuxfs2

[rkoster]

When using the cflinuxfs2 stack I'm running into issues with the ruby buildpack (v1.6.5).
The error below is raised because it expects the CF_STACK env far to be set to cflinuxfs2.
I was able to verify the this by ENV['CF_STACK'] = 'cflinuxfs2' to the top of the compile script, which fixed the issue.

2015/08/22 13:10:23  Staging failed - have you added a buildpack for this type of application?
Starting the CloudRocker container...
Running Buildpacks...
It looks like you're deploying on a stack (currently set to *lucid64*) that's not supported by this buildpack.
That could be because you're using a recent buildpack release on a deprecated stack.
If you're using the buildpack installed by your CF admin, please let your admin know you saw this error message.
If you at one point specified a buildpack that's at git URL, please make sure you're pointed at a version that supports this stack.

 Failed to compile droplet: exit status 44