Merge #19613: travis: Re-enable s390x

fae656144e travis: Re-enable s390x (MarcoFalke)

Pull request description:

  According to travis, the issue has been solved. Quote

  > I would like to confirm that we have resolved this issue and most of our users are reported that this issue has been resolved on their end as well. Could you please re-check and see if that still exists for you?

ACKs for top commit:
  theStack:
    ACK fae656144e

Tree-SHA512: cf42f96d25474a9dcf0817a049e30e29714731d708f73c40a3042b0c70a71ff08f07dd96a89f0dcd5a50a63a355cf30b3511172a32b8af7d5a2e13ad222a4b49

.travis.yml

@@ -72,14 +72,12 @@ jobs:
         FILE_ENV="./ci/test/00_setup_env_arm.sh"
         QEMU_USER_CMD=""
 
-# s390 build was disabled temporarily because of disk space issues on the Travis VM
-#
-#    - stage: test
-#      name: 'S390x  [GOAL: install]  [buster]  [unit tests, functional tests]'
-#      arch: s390x  # Can disable QEMU_USER_CMD and run the tests natively without qemu
-#      env: >-
-#        FILE_ENV="./ci/test/00_setup_env_s390x.sh"
-#        QEMU_USER_CMD=""
+    - stage: test
+      name: 'S390x  [GOAL: install]  [buster]  [unit tests, functional tests]'
+      arch: s390x  # Can disable QEMU_USER_CMD and run the tests natively without qemu
+      env: >-
+        FILE_ENV="./ci/test/00_setup_env_s390x.sh"
+        QEMU_USER_CMD=""
 
     - stage: test
       name: 'Win64  [GOAL: deploy]  [unit tests, no gui, no functional tests]'