test suite: Arrange not to overwrite .save-env on t-chain-test

We want .save-env to be the env on entry to the original test, not
on execution of t-chain-test with a modified environment.

Without this the effects of t-defdistro might leak into the execution
of tests/setup/examplegit.  This is no good because
tests/setup/examplegit relies in the -dtest-dummy which is our usual
dgit runes.  (Or to put it another way, examplegit uses many suites
which are not (yet) set up properly by t-defdistro.)

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>