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>

.forgejo/workflows/nix-shell.yaml

@@ -0,0 +1,39 @@
+---
+name: test nix/shell
+
+on:
+  push:
+
+jobs:
+  nix/shell:
+    runs-on: nixos-flakes
+    steps:
+      - name: checkout
+        uses: actions/checkout@v3
+
+      - name: run a command within a Nix shell
+        uses: ./shell
+        with:
+          flake: ./shell/test
+          package: "hello"
+          run: |
+            hello
+
+      - name: ensure we're not in a dev environment
+        uses: ./shell
+        with:
+          flake: ./shell/test
+          package: "hello"
+          run: |
+            set -x
+
+            # if we have gcc installed, we're in a dev environment.
+            # we don't want to be in a dev environment
+            type -p gcc >/dev/null && exit 1
+            hello
+
+      - name: no commands should still work
+        uses: ./shell
+        with:
+          flake: ./shell/test
+          package: "hello"