fix systemd hardening eval errors

[brianmcgillion]

Description of changes

fix eval errors

sudo rm -fr /tmp/tempstore && mkdir /tmp/tempstore && chmod 777 /tmp/tempstore

nix flake check --no-build --debug -L --no-eval-cache --store /tmp/tempstore --show-trace

leeds to this

 … while calling the 'import' builtin
         at /tmp/tempstore/nix/store/kshrl9wlzk8p0yq7xlym31nzzsyblb4p-source/modules/common/systemd/harden.nix:15:32:
           14|       // lib.optionalAttrs (!builtins.elem "${svc}.service" cfg.excludedHardenedConfigs) {
           15|         ${svc}.serviceConfig = import "${configs-dir}/${svc}.nix";
             |                                ^
           16|       }

fixed by removing builtins.readFile

Checklist for things done

• Summary of the proposed changes in the PR description
• More detailed description in the commit message(s)
• Commits are squashed into relevant entities - avoid a lot of minimal dev time commits in the PR
• Contribution guidelines followed
• Ghaf documentation updated with the commit - https://tiiuae.github.io/ghaf/
• PR linked to architecture documentation and requirement(s) (ticket id)
• Test procedure described (or includes tests). Select one or more:
  • Tested on Lenovo X1 x86_64
  • Tested on Jetson Orin NX or AGX aarch64
  • Tested on Polarfire riscv64
• Author has run make-checks and it passes
• All automatic Github Action checks pass - see actions
• Author has added reviewers and removed PR draft status
• Change requires full re-installation
• Change can be updated with nixos-rebuild ... switch

Instructions for Testing

• List all targets that this applies to:
• Is this a new feature
  • List the test steps to verify:
• If it is an improvement how does it impact existing functionality?
  fixing eval errors