testing/singularity: disable temporarily

Build fails on the builders with caching errors and `no rule to make
target cni/dhcp`. Unfortunately, this cannot be reproduced locally.

Someone with accesses to the builders must investigate this further.
Until then, disable it to prevent the builders from repeatably
attempting rebuilds.

See also: #10737

testing/singularity/APKBUILD

@@ -5,7 +5,7 @@ pkgver=3.3.0
 pkgrel=3
 pkgdesc="Application containers focused on reproducibility for scientific computing and HPC world."
 url="https://www.sylabs.io/singularity/"
-arch="all"
+arch=
 license="BSD-3-Clause BSD-3-Clause-LBNL"
 options="suid !check" # no test suite from upstream
 depends="squashfs-tools"