bug#32539: Cuirass: a commit that updates doc will produce 86 tests

[Clément Lassieur]

Evaluation 338[1] is triggered by commit 1d8d69c[2], which only updates
the documentation.  This evaluation produces 86 useless tests:
test.mcron.i686-linux, test.opensmtpd.i686-linux, etc.  I believe those
tests are triggered at each evaluation (almost), and it's a considerable
waste of resources.

[1]: https://berlin.guixsd.org/eval/338
[2]: 
https://git.savannah.gnu.org/cgit/guix.git/commit/?id=1d8d69c863ae88d5fc20e52e49a22f68c531c1db