config: pipeline: fix build-only-trees rules

We have a template for restricting build-only jobs to a list of allowed
trees only. However, this anchor contained the `rules` top-level key
itself, meaning using it as `rules: *build-only-trees-rules` would end
up creating the following object:

```
  kbuild-job:
    ...
    rules:
      rules:
        tree:
          ...
```

The nested `rules` keys prevented successful parsing of those rules,
which were therefore ignored, causing the corresponding job to run on
all trees.

Signed-off-by: Arnaud Ferraris <arnaud.ferraris@collabora.com>

config/pipeline.yaml

@@ -97,15 +97,14 @@ _anchors:
 
 ### Frequently used rules
 
- build-only-trees: &build-only-trees-rules
- rules:
- tree:
- - 'amlogic'
- - 'ardb'
- - 'arnd'
- - 'clk'
- - 'efi'
- - 'peterz'
+ build-only-trees-rules: &build-only-trees-rules
+ tree:
+ - 'amlogic'
+ - 'ardb'
+ - 'arnd'
+ - 'clk'
+ - 'efi'
+ - 'peterz'
 
 api: