install_deps.sh: fix format, add NixOS support

Signed-off-by: Jakub Sokołowski <jakub@status.im>

_assets/scripts/install_deps.sh

@@ -1,13 +1,14 @@
 #!/usr/bin/env bash
 
-if [ -x "$(command -v apt)" ]; then
+if [[ -x $(command -v apt) ]]; then
   apt install -y protobuf-compiler jq
-fi
-
-if [ -x "$(command -v pacman)" ]; then
+elif [[ -x $(command -v pacman) ]]; then
   pacman -Sy protobuf jq --noconfirm
-fi
-
-if [ -x "$(command -v brew)" ]; then
+elif [[ -x $(command -v brew) ]]; then
   brew install protobuf jq
+elif [[ -x $(command -v nix-env) ]]; then
+  nix-env -iA nixos.protobuf3_13
+else
+  echo "ERROR: No known package manager found!" >&2
+  exit 1
 fi