Cannot concretize with `config:install_missing_compilers:true`

[Massimiliano Culpo]

Steps to reproduce

Trying to concretize a simple environment like:

spack:
  specs:
  - zlib-ng %gcc@=13.2.0
  view: true
  config:
    install_missing_compilers: true
  concretizer:
    unify: true

fails with a missing compiler error.

Error message

==> Error: concretization failed for the following reasons:

   1. No valid version for 'zlib-ng' compiler 'gcc' satisfies '@13'

Information on your system

• Spack: 0.22.0.dev0 (9d86633)
• Python: 3.11.5
• Platform: linux-ubuntu20.04-icelake
• Concretizer: clingo

General information

• I have run spack debug report and reported the version of Spack/Python/Platform
• I have searched the issues of this repo and believe this is not a duplicate
• I have run the failing commands in debug mode and reported the output

[Harmen Stoppels]

Closed by #43876