fix bug with module detection when running buildtest config validate

[shahzebsiddiqui]

@wyphan i tested this now in the container its working

(buildtest)  ~/Documents/github/buildtest/ [fix_module_detection_bug] docker run -it -v  $BUILDTEST_ROOT:/home/spack/buildtest ghcr.io/buildtesters/buildtest_spack:latest
==> Regenerating tcl module files
spack@89d9d1804325:~/buildtest$ source scripts/spack_container/setup.sh 
(buildtest) spack@89d9d1804325:~/buildtest$ buildtest cg validate
/home/spack/buildtest/buildtest/settings/spack_container.yml is valid

I can see the executors

(buildtest) spack@89d9d1804325:~/buildtest$ buildtest cg executors
generic.local.bash
generic.local.sh