After further investigation, I believe the problem is that the
official nightly packages were built on an outdated version of openSUSE
Tumbleweed (from 2019-11-06) that uses Perl 5.28.1, and the dependency
on this version of Perl is essentially hard-coded (or more precisely,
it is automatically set at build time). Tumbleweed has since moved to
Perl 5.30.1, and so the existing X2Go RPMs cannot be installed on the
current Tumbleweed release.

The solution is to upgrade whatever system was used to produce the
official X2Go nightly packages to the latest version of Tumbleweed and
then rebuild the X2Go RPMs.  Or alternatively, use the openSUSE Build
Service (OBS) to build the packages; this will take care of
automatically rebuilding the packages according to the latest version
of Tumbleweed.

Until the X2Go developers implement one of these two solutions, I have
implemented the second solution myself.  I have created an OBS project
for x2goclient, x2goclient, and pinentry-x2go, and contributed packages
using the latest SCM commits:
<https://build.opensuse.org/project/show/home:psych0naut:x2go>

Note that I do not have any sort of automated system in place for
keeping these builds in sync with the latest commits to X2Go.  I
consider my builds to be a stopgap measure until the X2Go developers
fix their own nightly build system.