Bug 1655669 - part 2: Install snap binary because it's now required by snapcraft r=sfraser

It became mandatory in https://github.com/snapcore/snapcraft/pull/3173/files#diff-84615e6204c0bf79064d03915b64c656R196

Differential Revision: https://phabricator.services.mozilla.com/D85151

taskcluster/docker/firefox-snap/Dockerfile

@@ -13,7 +13,7 @@ ENV LANG='en_US.UTF-8' \
     SNAP_ARCH='amd64' \
     TERM='dumb'
 
-# Grab dependencies
+# Grab dependencies. snapd is now required per https://github.com/snapcore/snapcraft/pull/3210
 RUN apt-get update && \
     apt-get dist-upgrade --yes && \
     apt-get install --yes \
@@ -27,7 +27,9 @@ RUN apt-get update && \
       git \
       python3 \
       locales \
-      sudo && \
+      snapd \
+      sudo \
+      && \
     apt-get clean && \
 locale-gen "$LANG"