Update input description and add test workflow

.github/workflows/ci.yml

@@ -211,3 +211,13 @@ jobs:
         with:
           duration: 5m
           authorized-users: grahamc
+
+  install-with-non-default-inputs:
+    name: Install Nix using non-default Action inputs
+    runs-on: ubuntu-22.04
+    steps:
+      - uses: actions/checkout@v4
+      - uses: DeterminateSystems/nix-installer-action@main
+        with:
+          source-url: https://install.determinate.systems/nix-installer/stable/X64-Linux
+      - run: nix --version

action.yml

@@ -92,7 +92,7 @@ inputs:
     description: The tag of `nix-installer` to use (conflicts with `source-revision`, `source-branch`, `source-pr`)
     required: false
   source-url:
-    description: A URL pointing to a `nix-installer.sh` script
+    description: A URL pointing to a `nix-installer` executable
     required: false
   nix-package-url:
     description: The Nix package URL