Include root in returned car file

[willscott]

@rvagg noted that #355 only puts a root in a temporary car used for caching.

if we are seeing CARs come out without roots then (a) this PR is unlikely to impact that and (b) it's a genuine bug with changes we've made recently that probably only shows up in a particular mode that we may not be testing in integration tests.

[rvagg]

So far investigation has yielded no clues as to how Lassie might be producing root-less CARs, even looking through historical versions of Lassie, only more questions that lead to suspecting the problems are elsewhere .. we're just unsure where!

[hannahhoward]

Closing as we've identified the issue as out of lassie