cmd-build: skip-compression on manifest json

I noticed this when running some build tests:

```
+ cosa compress
Targeting build: 41.20241112.20.0
Compressing: builds/41.20241112.20.0/x86_64
INFO - Running command: ['xz', '-c9', '-T6', 'builds/41.20241112.20.0/x86_64/fedora-coreos-41.20241112.20.0-ostree.x86_64-manifest.json']
Compressed: fedora-coreos-41.20241112.20.0-ostree.x86_64-manifest.json.xz
```

Which I don't think we really want.

Also let's make the spacing the same as the previous entry in the JSON.

src/cmd-build

@@ -571,9 +571,10 @@ cat > tmp/images.json <<EOF
         "skip-compression": true
     },
     "oci-manifest": {
-      "path": "${ostree_oci_manifest_path}",
-      "sha256": "${ostree_oci_manifest_sha256}",
-      "size": ${ostree_oci_manifest_size}
+        "path": "${ostree_oci_manifest_path}",
+        "sha256": "${ostree_oci_manifest_sha256}",
+        "size": ${ostree_oci_manifest_size},
+        "skip-compression": true
     }
   }
 }