ci: fix formatting check erroring out in an incorrect manner

[lineargraph]

previously this would fail with a non descript openTempFileWithDefaultPermissions: permission denied (Permission denied)

now the correct file gets identified in the error log

[lineargraph]

quoting https://github.com/Lassulus/wrappers/actions/runs/25938713491/job/76250654379?pr=163

       Output paths:
         /nix/store/5rxrdqviy44x0z7dq0i4546mg84hwkza-formatting-check
       Last 5 log lines:
       > ERRO file has changed path=checks/formatting.nix prev_size=395 prev_mod_time="2026-05-15 20:03:59 +0000 UTC" current_size=388 current_mod_time="2026-05-15 20:04:00 +0000 UTC"
       > traversed 109 files
       > emitted 103 files for processing
       > formatted 103 files (1 changed) in 995ms
       > Error: unexpected changes detected, --fail-on-change is enabled
       For full logs, run:

[zimward]

yup that makes the CI fail a lot clearer