feat: explicitly don't install_missing_compilers

This does not represent any change, but is mainly intended to be explicit and to allow for testing with this value set to true.

spack-environment/dev/spack.yaml

@@ -2,6 +2,8 @@ spack:
   include:
   - ../concretizer.yaml
   - ../packages.yaml
+  config:
+    install_missing_compilers: false
   specs:
     - acts
     - actsvg