Release 1.2.1

[geofft]

Bump nix dependency to 0.21.2 to pick up fix for nix-rust/nix#1541 aka RUSTSEC-2021-0119, memory corruption when using initgroups on a user in more than 16 groups

Also add an integration regression test.

[geofft]

OK, finally got the failing test working: https://github.com/twosigma/nsncd/runs/3833571070

I originally tried to be lazy and add it to /etc/group, but reliably detecting "did nsncd crash" ended up being hard (I think we were racing the nsncd process in the middle of crashing, or something?). So I instead just went with extending our existing whatami module to dynamically create am_i_foo users and make it so that this user is in 20 groups if the current process name is foo. So we check for the group membership of am_i_nsncd, which works iff we routed the lookup through nsncd.

[blinsay]

since we're accepting more names, should we set result->pw_name = name? it probably doesn't matter for this test but maybe we won't regret it later.

[leifwalsh]

I have...questions...about the implementation of _nss_whatami_initgroups_dyn, but also, if the test works it works and maybe that's okay.

Let me know if you'd like me to comment all over that function, or just merge.

[geofft]

I'm going to merge (and do an internal release) because both of these are comments on the test code, but definitely happy to accept comments on the test code. I am not sure I have a lot of answers for questions though :)