upgrade nix to 0.31

[alexanderkjall]

No description provided.

[weihanglo]

Test failed. Mind talking a look?

[decathorpe]

Also looking at this for Fedora. It seems the error is unrelated to either jobserver or nix:

fnmatch.c:124:14: error: conflicting types for 'getenv'; have 'char *(void)'
  124 | extern char *getenv ();
      |              ^~~~~~
In file included from fnmatch.c:40:
/usr/include/stdlib.h:53:7: note: previous declaration of 'getenv' with type 'char *(const char *)'
   53 | char *getenv (const char *);
      |       ^~~~~~
fnmatch.c: In function 'internal_fnmatch':
fnmatch.c:273:33: error: too many arguments to function 'getenv'; expected 0, have 1
  273 |               posixly_correct = getenv ("POSIXLY_CORRECT") != NULL ? 1 : -1;
      |                                 ^~~~~~  ~~~~~~~~~~~~~~~~~
fnmatch.c:124:14: note: declared here
  124 | extern char *getenv ();
      |              ^~~~~~

This seems to be a compilation failure of make 4.4.1 on alpine (not sure why make is compiled during CI).