Fix mmdebstrap chroot creation when building for experimental

Reinhard Tartlerrequested to merge
siretart/pipeline:siretart/mmdebstrap into master

When creating a mmdebstrap for experimental, the tarball gets created against "$DEFAULT_RELEASE", because 'experimental' is frequently not installable. This this causes the tarball to be created in a location that doesn't match the later 'sbuild' command. This causes a second chroot tarball to be created, but that one doesn't contain the customizations such as adding the 'ca-certificates' package. This is avoided by creating a symlink.

Fixes: #527

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