Build.snapcraft.io: Support ccache cache

[林博仁(Buo-ren Lin)]

I'd like to ask for the support of ccache cache in build.snapcraft.io as it can minimize build time.

Travis CI managed to achieve this by packing and uploading the cache somewhere after building and pull it back at the start of the build:

https://docs.travis-ci.com/user/caching#How-does-caching-work

Their implementation appears to be:

https://github.com/travis-ci/casher

and has set a 512MiB cache size limit in the build environment.

Also quoting @cjwatson 's comment about this subject:

> That said, my instinct is that anything like this would require a great deal of care to
> avoid accidentally creating security vulnerabilities due to multi-tenanting and
> information leaks between builds via ccache data; it would be a large project to do this
> safely, and until we could be absolutely confident in its safety, it’s more valuable to
> have certainty that each build is strongly isolated from the next.