Get effective user in Nix commands

‘geteuid’ gives us the user that the command is being run as,
including in setuid modes. By using geteuid to determind id, we can
avoid the ‘sudo -i’ hack when upgrading Nix. So now, upgrading Nix on
macOS is as simple as:

$ sudo nix-channel --update
$ sudo nix-env -u

$ sudo launchctl stop org.nixos.nix-daemon
$ sudo launchctl start org.nixos.nix-daemon
or
$ sudo systemctl restart nix-daemon

src/libutil/util.cc

@@ -468,7 +468,7 @@ static Lazy<Path> getHome2([]() {
  std::vector<char> buf(16384);
  struct passwd pwbuf;
  struct passwd * pw;
- if (getpwuid_r(getuid(), &pwbuf, buf.data(), buf.size(), &pw) != 0
+ if (getpwuid_r(geteuid(), &pwbuf, buf.data(), buf.size(), &pw) != 0
  || !pw || !pw->pw_dir || !pw->pw_dir[0])
  throw Error("cannot determine user's home directory");
  homeDir = pw->pw_dir;

src/nix-channel/nix-channel.cc

@@ -169,7 +169,7 @@ int main(int argc, char ** argv)
 
  // Figure out the name of the channels profile.
  ;
- auto pw = getpwuid(getuid());
+ auto pw = getpwuid(geteuid());
  std::string name = pw ? pw->pw_name : getEnv("USER", "");
  if (name.empty())
  throw Error("cannot figure out user name");