Jenkinsfile: use buildx on s390x and ppc64le

Commit 1be2cc2568ddd52a3b27d607a9c9f7c9347d3c50 updated the
Makefile to force the use of BuildKit, if `USE_BUILDX` was
not set.

As a side-effect, Jenkins now started using BuildKit on
s390x and ppc64le as well, because it overwrote the
`DOCKER_BUILDKIT=0` that was set.

This commit forces the use of buildx on s390x and ppc64le.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Jenkinsfile

@@ -375,8 +375,12 @@ pipeline {
                         expression { params.s390x }
                     }
                     agent { label 's390x-ubuntu-1604' }
-                    // s390x machines run on Docker 18.06, and buildkit has some bugs on that version
-                    environment { DOCKER_BUILDKIT = '0' }
+                    // s390x machines run on Docker 18.06, and buildkit has some
+                    // bugs on that version. Build and use buildx instead.
+                    environment {
+                        USE_BUILDX      = '1'
+                        DOCKER_BUILDKIT = '0'
+                    }
 
                     stages {
                         stage("Print info") {
@@ -484,8 +488,12 @@ pipeline {
                         expression { params.s390x }
                     }
                     agent { label 's390x-ubuntu-1604' }
-                    // s390x machines run on Docker 18.06, and buildkit has some bugs on that version
-                    environment { DOCKER_BUILDKIT = '0' }
+                    // s390x machines run on Docker 18.06, and buildkit has some
+                    // bugs on that version. Build and use buildx instead.
+                    environment {
+                        USE_BUILDX      = '1'
+                        DOCKER_BUILDKIT = '0'
+                    }
 
                     stages {
                         stage("Print info") {
@@ -569,8 +577,12 @@ pipeline {
                         expression { params.ppc64le }
                     }
                     agent { label 'ppc64le-ubuntu-1604' }
-                    // ppc64le machines run on Docker 18.06, and buildkit has some bugs on that version
-                    environment { DOCKER_BUILDKIT = '0' }
+                    // ppc64le machines run on Docker 18.06, and buildkit has some
+                    // bugs on that version. Build and use buildx instead.
+                    environment {
+                        USE_BUILDX      = '1'
+                        DOCKER_BUILDKIT = '0'
+                    }
 
                     stages {
                         stage("Print info") {
@@ -678,8 +690,12 @@ pipeline {
                         expression { params.ppc64le }
                     }
                     agent { label 'ppc64le-ubuntu-1604' }
-                    // ppc64le machines run on Docker 18.06, and buildkit has some bugs on that version
-                    environment { DOCKER_BUILDKIT = '0' }
+                    // ppc64le machines run on Docker 18.06, and buildkit has some
+                    // bugs on that version. Build and use buildx instead.
+                    environment {
+                        USE_BUILDX      = '1'
+                        DOCKER_BUILDKIT = '0'
+                    }
 
                     stages {
                         stage("Print info") {