Set maximum name length in Nix

Previously we allowed any length of name for Nix derivations. This is
bad because different file systems have different max lengths. To make
things predictable, I have picked a max. This was done by trying to
build this derivation:

  derivation {
    name = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa";
    builder = "/no-such-path";
    system = "x86_64-linux";
  }

Take off one a and it will not lead to file name too long. That ends
up being 212 a’s. An even smaller max could be picked if we want to
support more file systems.

Working backwards, this is why:

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa-${name}.drv.chroot

> 255 - 32 - 1 - 4 - 7 = 211

0 files changed, 0 insertions, 0 deletions