Specify /bin/bash as the tinypilot user's shell (#1261)

Without a shell specified, the user defaults to /usr/sbin/nologin, which
prevents us from executing commands as the tinypilot user through sudo
su.

I tested this on a clean install of Raspbian Buster Lite, and it fixes
the `sudo su` issue.

Fixes #1260

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debian-pkg/debian/tinypilot.postinst

@@ -21,6 +21,9 @@ getent group "${TINYPILOT_GROUP}" > /dev/null || \
 # adduser is idempotent, so we don't need to check existence first.
 adduser \
   --system \
+  `# We have to specify a shell to override the default /usr/sbin/nologin, ` \
+  `# which prevents us from executing commands through "sudo su".` \
+  --shell /bin/bash \
   --ingroup "${TINYPILOT_GROUP}" \
   --home "${TINYPILOT_HOME_DIR}" \
   "${TINYPILOT_USER}"