Use core22 for snapcraft image

Dockerfile.snap

@@ -14,11 +14,11 @@ RUN apt-get install --yes \
       jq \
       squashfs-tools
 
-# Grab the core20 snap (which snapcraft uses as a base) from the stable channel
+# Grab the core22 snap (which snapcraft uses as a base) from the stable channel
 # and unpack it in the proper place.
-RUN curl -L $(curl -H 'X-Ubuntu-Series: 16' 'https://api.snapcraft.io/api/v1/snaps/details/core20' | jq '.download_url' -r) --output core20.snap
-RUN mkdir -p /snap/core20
-RUN unsquashfs -d /snap/core20/current core20.snap
+RUN curl -L $(curl -H 'X-Ubuntu-Series: 16' 'https://api.snapcraft.io/api/v1/snaps/details/core22' | jq '.download_url' -r) --output core22.snap
+RUN mkdir -p /snap/core22
+RUN unsquashfs -d /snap/core22/current core22.snap
 
 # Grab the snapcraft snap from the $RISK channel and unpack it in the proper
 # place.
@@ -61,7 +61,7 @@ RUN echo 'def new_patch(self, *, elf_file):' >> ${ELF_PY} && \
 # Multi-stage build, only need the snaps from the builder. Copy them one at a
 # time so they can be cached.
 FROM ubuntu:$UBUNTU
-COPY --from=builder /snap/core20 /snap/core20
+COPY --from=builder /snap/core22 /snap/core22
 COPY --from=builder /snap/snapcraft /snap/snapcraft
 COPY --from=builder /snap/bin/snapcraft /snap/bin/snapcraft
 COPY --from=builder /opt/patchelf/bin/patchelf /snap/snapcraft/current/bin