By default `libnss-unknown` violates the assumption that the result of
calling `getpwent()`/`getpwuid()` does not change when the environment
changes as the value for the `pw_dir` field depends on the `$HOME` var.

For instance the `posix/wordexp-test` test in the glibc testsuite
breaks with `libnss-unknown`:

* the call to `getpwuid()` with `HOME=/dummy/home` returns
  `/dummy/home` due to `libnss-unknown`
* `unsetenv ("HOME")` is called
* any call to `getpwuid()` will now return `/` due to
  `libnss-unknown` as no `$HOME` is set
* test fails because results don't match

Setting `NSS_UNKNOWN_HOME` before running the test will get rid of the
dependency between `getpwuid()` and `$HOME`.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>