Hi Guix,

It appears that the attatched derivation to build webkitgtk using the 
following guix

```
Generation 2	Jun 26 2019 20:21:09	(current)
   guix ba91247
     repository URL: https://git.savannah.gnu.org/git/guix.git
     branch: master
     commit: ba912479e8481d69b699ce348b35d5d70f0c9367
```

fails to build in a way that looks like a race. The suspicious lines in 
the attatched log are:

```
Traceback (most recent call last):
   File "/tmp/guix-build-webkitgtk-2.24.2.drv-0/webkitgtk-2.24.2/Source/WebKit/Scripts/generate-message-receiver.py", line 28, in <module>
     import webkit.messages
EOFError: EOF read where object expected
```

I have been able to sucessfully build webkitgtk on multiple hosts by using 
`-c 2`

Best,
Jack