Update nix to 0.14 to fix compilation error

[lautis]

On armv7 and some other architectures, compiling nix 0.12 / latest libc errors with

error[E0425]: cannot find value `PTRACE_SETFPXREGS` in module `libc`
  --> /cargo/registry/src/github.com-1ecc6299db9ec823/nix-0.12.0/src/sys/ptrace/linux.rs:85:9
   |
85 |         PTRACE_SETFPXREGS,

Nix 0.14 contains a fix for this.