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/action.yml

@@ -0,0 +1,25 @@
+---
+name: nix-shell
+author: Gergely Nagy
+description: "Run commands within a Nix development environment"
+inputs:
+  flake:
+    description: "The flake to use for the development environment"
+    default: "."
+    required: true
+  package:
+    description: "The package to use for the development environment"
+    required: false
+  run:
+    description: "Commands to run within the development environment"
+    required: false
+runs:
+  using: "composite"
+  steps:
+    - id: nix-shell
+      shell: bash
+      run: $GITHUB_ACTION_PATH/bin/nix-shell
+      env:
+        INPUT_FLAKE: ${{ inputs.flake }}
+        INPUT_PACKAGE: ${{ inputs.package }}
+        INPUT_RUN: ${{ inputs.run }}