ci: travis: switch coverity build back to Fedora 31 image

While it builds there is a problem to pick up the analyzing units for
the scanner. My best guess is that gcc10 is to new and we need to wait
for the next cov-tools release before we can bump this.

.travis.yml

@@ -71,7 +71,7 @@ jobs:
       env: DISTRO=Debian96
     - os: linux
       if: type = cron
-      env: DISTRO=Fedora32 CI_BUILD_TYPE=coverity
+      env: DISTRO=Fedora31 CI_BUILD_TYPE=coverity
     - os: linux
       if: type = cron
       env: DISTRO=Fedora32 CI_BUILD_TYPE=asan