ccache can help greatly in speeding up recompilations in sbuild, but it
is a bit annoying to configure at each schroot creation.
So, add --cache option (and relative CCACHE config parameter) to
configure ccache for sbuild schroots.
By default we use a shared ccache directory, but each schroot can use
a customized one if needed (with local parameters) by using --cache-dir
(or CCACHE_DIR).
Default ccache max-size is 4G, but can be configured with --ccache-size
(or CCACHE_SIZE), the size value is applied to each ccache path, so can
be shared by multiple schroots or applied to a single one.
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