setup.sh: use six X characters in mktemp invocation

[ghost]

Description of changes

Closes #178625

The busybox version of mktemp requires exactly six X characters in the argument to mktemp, unlike the coreutils version of mktemp.

Let's accomodate packages, like epson-escpr2, which fool setup.sh into using the busybox version instead of the stdenv version.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 22.11 Release Notes (or backporting 22.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[Mindavi]

https://refspecs.linuxbase.org/LSB_3.1.0/LSB-Core-generic/LSB-Core-generic/mktemp.html

The template should have at least six trailing 'X' characters. These characters are replaced with characters from the portable filename character set in order to generate a unique name.

Looks good to me. I like that stdenv is made to be very portable, that will make porting easier if some other shell is used in the future.

[ghost]

https://refspecs.linuxbase.org/LSB_3.1.0/LSB-Core-generic/LSB-Core-generic/mktemp.html

The template should have at least six trailing 'X' characters.

One of the great parts about contributing to nixpkgs is learning unexpected stuff like this :)

I like that stdenv is made to be very portable, that will make porting easier if some other shell is used in the future.

Well, technically coreutils is part of stdenv... in fact that was the main reason why I did not test my changes on any other shell or "utils".

I think we ought to alert people to the fact that setup.sh currently works with busybox, and there are packages which depend on this behavior.

One way to do this is to make sure that applications/misc/hello/default.nix fails to build if people don't preserve this behavior. For example, hello deliberately passes a function (rather than an attrset) to mkDerivation so people are forced to make sure they don't break that capability. Unfortunately making hello use busybox to execute setup.sh would add an unnecessary and large dependency to hello, so that isn't a good idea.

Any ideas on how else to make sure people notice if they make changes to setup.sh that don't work with busybox?