fetch.sh: provide dependencies using nix-shell if available

tools/FetchMacOS/fetch.sh

@@ -7,6 +7,12 @@ set +x;
 SCRIPTDIR="$(dirname "$0")";
 cd "$SCRIPTDIR"
 
+if [ -x "$(command -v nix-shell)" ]; then
+    echo "Nix detected, using nix-shell for dependencies..."
+    nix-shell --pure --run "python fetch-macos.py $*"
+    exit
+fi
+
 initpip() {
     if [ -x "$(command -v easy_install)" ]; then
         sudo easy_install pip

tools/FetchMacOS/shell.nix

@@ -0,0 +1,9 @@
+{ pkgs ? import <nixpkgs> {} }:
+
+pkgs.mkShell {
+  propagatedBuildInputs = with pkgs.python3Packages; [
+    python
+    requests
+    click
+  ];
+}