Build doc: document the need for sudo (Closes: #15055).

wiki/src/contribute/build.mdwn

@@ -36,13 +36,15 @@ To build Tails you need:
            vmdebootstrap && \
        sudo systemctl restart libvirtd
 
-2. Ensure your user is in the relevant groups:
+2. Ensure your user can run commands as root with `sudo`.
+
+3. Ensure your user is in the relevant groups:
 
        for group in kvm libvirt libvirt-qemu ; do
           sudo adduser "$(whoami)" "$group"
        done
 
-3. Logout and log back in to apply the new group memberships.
+4. Logout and log back in to apply the new group memberships.
 
 # Build Tails