The UnkeyedValidPathInfo test was testing an ancient version but not the
current version. Doesn't make much sense to me.
Change-Id: Ib476a4297d9075f2dcd31a073b3e7b149b2189af
this lets us set per-test-program environment variables rather than only
a single, global default. this was supported in nix originally but
might've gone partially missing in the upstream backports process?
Change-Id: Iad0919841b1b6d11e0b7ebd3920449a62f544e77