Fix handling of cowbuilder update lock

[mark0n]

A left behind lock file does not mean the resource is still locked. Bail out if lock cannot be acquired. This fixes #158.

[zeha]

This changes the behaviour though: in those 10 minutes, no other builds will run, even if for most builds waiting for the update does not actually matter.

[mark0n]

We are building lots of applications that pull in libraries that have been build by JDG. If we wouldn't run apt-get update on the cowbuilder environment we would potentially build against old versions of our libraries which I consider wrong. This actually has practical relevance since in our case the repo job of the library triggers the source job of the application.

[zeha]

It is my understanding that apt-get update runs anyway, but /in/ the COW instance for this build. The lock is about the general chroot, where updating should only update the minimal Debian image.

[mark0n]

On our cluster it seems like apt-get update does not run as part of the cowbuilder_run (I guess I should see that in the build log, right?) - that's why we ran into this issue in the first place. Can you give me a hint how to enable this behavior?