Add license dir to config (#30135)

* Change license dir from hard-coded to a configurable item

* Change config item to be a string not an array

Co-authored-by: Todd Gamblin <tgamblin@llnl.gov>

etc/spack/defaults/config.yaml

@@ -33,6 +33,9 @@ config:
   template_dirs:
     - $spack/share/spack/templates
 
+  # Directory where licenses should be located
+  license_dir: $spack/etc/spack/licenses
+
   # Temporary locations Spack can try to use for builds.
   #
   # Recommended options are given below.