topgrade: gate neovim tarball updater to debian family only

update-neovim.sh and the topgrade [commands] entry that calls it are
only meaningful where neovim is not a distro package. On arch-family
boxes the template renders empty, so chezmoi removes the script
instead of deploying a no-op, and topgrade no longer runs a step that
could fail on unmanaged paths.

dot_local/bin/update-neovim.sh.tmpl

@@ -1,3 +1,4 @@
+{{- if eq .os_family "debian" }}
 #!/usr/bin/env bash
 # =============================================================================
 #  update-neovim.sh — chezmoi-managed, deployed to ~/.local/bin/update-neovim.sh
@@ -65,4 +66,5 @@ sudo tar -xzf "$TMP_TARBALL" -C /opt/
 sudo ln -sf "/opt/${NVIM_EXTRACT_DIR}/bin/nvim" /usr/local/bin/nvim
 
 echo "neovim $NVIM_TARGET_VERSION installed"
-nvim --version | head -1
+nvim --version | head -1
+{{- end }}