Enable ccache on upstream-merger builds, allow sharing with developers

[Francis Ginther]

According to [1] shared ccache should be possible. We've had trouble with this in the past related to files being removed by one pbuilder instance while another was attempting to access them [2], [3].

Ideally we'd like to leverage the ccache to speed up builds for developers at home. Not sure if this will work yet, but it's worth exploring.

[1] - http://ccache.samba.org/manual.html#_sharing_a_cache
[2]
-------------------------------snip---------------------------
I: Setting up ccache
chown: changing ownership of
`/var/cache/pbuilder/build//47537//var/cache/pbuilder/ccache/9/stats.lock':
No such file or directory
I: unmounting results filesystem
I: unmounting /var/cache/pbuilder/ccache filesystem
I: unmounting dev/pts filesystem
I: unmounting proc filesystem
I: cleaning the build env
I: removing directory /var/cache/pbuilder/build//47537 and its
subdirectories
ERROR:pbuilderjenkins:Error during build execution
ERROR:pbuilderjenkins:pdebuild failed
pbuilder returned ERROR.
Build step 'Debian PBuilder NG' marked build as failure
Sending e-mails to: <email address hidden>
<email address hidden>
Build was marked for publishing on https://jenkins.qa.ubuntu.com/
Finished: FAILURE
-------------------------------snip---------------------------

[3] - http://www.netfort.gr.jp/~dancer/software/pbuilder-doc/pbuilder-doc.html#ccache