slurm: add alias attribute for backward compatibility

[veprbl]

Fixes: b917a87 ('slurm: 15-08-5-1 -> 17.02.6, slurm-llnl -> slurm')

Motivation for this change

Do as best as we can in absence of #19315

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

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