sbuild autopkgtest just skips the unshare tests

sbuild autopkgtest test currenly looks like this:

2429s build-procenv PASS
2429s unshare SKIP exit status 77 and marked as skippable
2429s unshare-qemuwrapper SKIP exit status 77 and marked as skippable

The unshare autopkg test is skipping because there is a check to see if unshare is working.
This fails like so:
unshare(CLONE_NEWNS|CLONE_NEWUSER) = -1 EACCES (Permission denied)

With that failure, the unshare test realizes the problem and exits 77, skipping the test.

In situations where this has regressed, that EACCES is due to LP: #2035315. Sbuild autopkgtest has an attempt at mitigating unshare problems by running the test again - this is the unshare-qemuwrapper. However this exits 77 for the same reason, which means that neither version of the unshare test actually runs.

Sample log:
https://autopkgtest.ubuntu.com/results/autopkgtest-noble/noble/amd64/s/sbuild/20240108_203545_af566@/log.gz