Add -type f to generate-matrix default find(1) options

This ensures we only list regular files and not symbolic links. It may
be the case that a project symlinks `stack.yaml` to one of the specific
configurations. In such cases, only the actual files should generate
matrix elements. Generating one for `stack.yaml` would be redundant.

Projects that attach extra tasks (weeder, coverage) to only the
`stack.yaml` build should either not use a symlink, or adjust the `find`
input accordingly.

generate-matrix/action.yml

@@ -8,7 +8,7 @@ inputs:
   find-options:
     description: "Arguments to find(1) stack-yaml files"
     required: true
-    default: "-maxdepth 1 -name 'stack*.yaml'"
+    default: "-type f -maxdepth 1 -name 'stack*.yaml'"
 outputs:
   stack-yamls:
     description: Version-sorted list of all files matching stack*.yaml