schema/compilers.py: fix validation of 2+ entries (#40627)

Fix the following syntax which validates only the first array entry:

```python
"compilers": {
    "type": "array",
    "items": [
        {
            "type": ...
        }
    ]
}
```

to

```python
"compilers": {
    "type": "array",
    "items": {
        "type": ...
    }
}
```

which validates the entire array.

Oops...

lib/spack/spack/schema/compilers.py

@@ -14,8 +14,7 @@
 properties = {
     "compilers": {
         "type": "array",
-        "items": [
-            {
+        "items": {
             "type": "object",
             "additionalProperties": False,
             "properties": {
@@ -69,8 +68,7 @@
                     },
                 }
             },
-            }
-        ],
+        },
     }
 }