Add a nix/shell action

Since it's basically the same as `nix/develop`, with only the nix
command changed, leverage the existing script.

Signed-off-by: Gergely Nagy <me@gergo.csillger.hu>

shell/test/flake.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1685566663,
+        "narHash": "sha256-btHN1czJ6rzteeCuE/PNrdssqYD2nIA4w48miQAFloM=",
+        "owner": "nixos",
+        "repo": "nixpkgs",
+        "rev": "4ecab3273592f27479a583fb6d975d4aba3486fe",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nixos",
+        "ref": "23.05",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

shell/test/flake.nix

@@ -0,0 +1,11 @@
+{
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/23.05";
+  };
+
+  outputs = { self, nixpkgs }:
+    let pkgs = nixpkgs.legacyPackages.x86_64-linux;
+    in {
+      packages.x86_64-linux.hello = pkgs.hello;
+    };
+}