Update the upstream Nix version on release proposal

Author: grahamc

.github/workflows/propose-release.yml

@@ -10,6 +10,9 @@ on:
       determinate-nix-version:
         type: string
         required: true
+      upstream-nix-version:
+        type: string
+        required: true
 
 concurrency:
   group: ${{ github.workflow }}
@@ -36,5 +39,6 @@ jobs:
         git commit -m "Update test fixtures with the new nix-installer version" || true
 
         sed -i 's#https://flakehub.com/f/DeterminateSystems/nix-src/=.*";#https://flakehub.com/f/DeterminateSystems/nix-src/=${{ inputs.determinate-nix-version }}";#' ./flake.nix
+        sed -i 's#https://releases.nixos.org/nix/nix-.*/nix-.*-";#https://releases.nixos.org/nix/nix-${{ inputs.upstream-nix-version }}/nix-${{ inputs.upstream-nix-version }}-";#' ./flake.nix
         git add flake.nix
         git commit -m "Update Determinate Nix release to ${{ inputs.determinate-nix-version }}" || true