bug#36419: webkitgtk build fails duo to suspected race

[Jack Hill]

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

4r0pcww7s08kpay74127z8k6zpcpnnr1-webkitgtk-2.24.2.drv
Description: Text document

0pcww7s08kpay74127z8k6zpcpnnr1-webkitgtk-2.24.2.drv.bz2
Description: Binary data