Caught by edef's harness. I rewrote the structured build log tests in
f2 and added a test for this one.
Before: it failed the build altogether:
```
building '/tmp/jade/pytest-of-jade/pytest-77/test_invalid_fields_files0_0/nix/store/skhjs5zs082nqiakj69qz32gyywz0v1a-unusual-loggi
ng.drv'...
error: unsupported JSON type 7
```
Now the output is as expected:
```
warning: Unable to handle a JSON message from the derivation builder: error: unsupported log field type number
@nix {"action": "start", "fields": [1.5], "id": 2, "type": 1, "level": 1, "text": "abc"}
```
Change-Id: Idbd28c5a4d4cf15f27207f83b730e7946a6a6964
Jade Lovelace