Fix typo in reference to step output

I renamed the references, but missed this one.

actionlint would have caught this.

```
.github/workflows/test_changes.yml:24:29: property "changed-installers" is not defined in object type {files: {conclusion: string; outcome: string; outputs: {string => string}}} [expression]
   |
24 |           CHANGED_FILES: ${{steps.changed-installers.outputs.all_changed_files}}
   |                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

Instead I detected it with debug logging in
[changes workflow run 37 attempt 2](https://github.com/iainelder/dotfiles/actions/runs/9780228648/job/27001451471).

Get changed installer files:

```
##[debug]Set output all_changed_files = programs/borgbackup/install.bash
```

Get changed installer names:

```
##[debug]Evaluating: steps.changed-installers.outputs.all_changed_files
...
##[debug]Result: null
...
##[debug]Set output changed_installers = []
```

.github/workflows/test_changes.yml

@@ -21,7 +21,7 @@ jobs:
       - name: Get changed installer names
         id: dirs
         env:
-          CHANGED_FILES: ${{steps.changed-installers.outputs.all_changed_files}}
+          CHANGED_FILES: ${{steps.files.outputs.all_changed_files}}
         run: |
           {
             printf "changed_installers="