pbuilder: add amd64

Until we have moved the builds build on arm01 so we can progress
towards fixing https://code.puri.sm/Librem5/image-builder/issues/50

install_jenkins.yml

@@ -44,6 +44,9 @@
         - dist: buster
           arch: armhf
           foreign: true
+        - dist: buster
+          arch: amd64
+          foreign: true
       tags:
         - gbp
     - role: scratchrepo