Unity: No repeated path for internal generated unity files

Fixes: #20002
21 jobs for unity-no-duplicate-path
Status Name Job ID Coverage
  External
passed cdash

passed cmake-dash3win10-windows-release-ninja

00:20:34

passed cmake-hythloth-linux-debug-makefiles+sphinx

00:45:28

passed cmake-minoc-linux-release-lint-iwyu-tidy

00:14:10

passed cmake-moonshade-osx-debug-ninja

00:15:37

passed cmake-moonshade-osx-release-makefiles

00:13:51

passed cmake-terfin-windows-release-vs2019

00:37:28

passed ghostflow-check-master

passed ghostflow-check-release

passed ghostflow-stager

passed cdash

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager