After further investigation, I believe the problem is that the official nightly packages were built on an outdated version of openSUSE Tumbleweed (from 2019-11-06) that uses Perl 5.28.1, and the dependency on this version of Perl is essentially hard-coded (or more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.