Build all snap targets on amd64, s390x or ppc64el

Set host architectures for building snaps to [amd64, s390x, ppc64el] and
build all target architectures [amd64, arm64, armhf, s390x, ppc64el].
diff --git a/snap/snapcraft.yaml b/snap/snapcraft.yaml
index 7aabfb3..4c81964 100755
--- a/snap/snapcraft.yaml
+++ b/snap/snapcraft.yaml
@@ -31,11 +31,16 @@
 confinement: strict
 
 architectures:
-  - build-on: amd64
-  - build-on: armhf
-  - build-on: arm64
-  - build-on: s390x
-  - build-on: ppc64el
+  - build-on: [amd64, s390x, ppc64el]
+    build-for: [amd64]
+  - build-on: [amd64, s390x, armhf, ppc64el]
+    build-for: [armhf]
+  - build-on: [amd64, s390x, arm64, ppc64el]
+    build-for: [arm64]
+  - build-on: [amd64, s390x, ppc64el]
+    build-for: [s390x]
+  - build-on: [amd64, s390x, ppc64el]
+    build-for: [ppc64el]
 
 parts:
   erl-iex: