fix(ci): move nix build -> nix-develop

This will trigger the nix build workflow, only on changes in the nix
files.

- This won't generate precompiled artifacts for any person wanting to
  use them from the main branch anymore.

  Also PR's that pass CI won't be able to be run with:
  ```
  nix run github:zellij-org/zellij/pr#
  ```
  without compilation anymore on any linux, or darwin system.

.github/workflows/nix-develop.yml

@@ -53,3 +53,20 @@ jobs:
           # If you chose API tokens for write access OR if you have a private cache
           authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
       - run: nix develop --profile devShell
+  build:
+    runs-on: ubuntu-latest
+    name: "nix build"
+    timeout-minutes: 35
+    steps:
+      - uses: actions/checkout@v3
+        with:
+          # Nix Flakes doesn't work on shallow clones
+          fetch-depth: 0
+      - uses: cachix/install-nix-action@v18
+      - uses: cachix/cachix-action@v12
+        with:
+          name: zellij
+          # If you chose API tokens for write access OR if you have a private cache
+          authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
+      - run: |
+          nix build || nix build --substituters 'https://cache.nixos.org' --extra-substituters ''

.github/workflows/nix.yml

@@ -1,28 +0,0 @@
-name: nix
-
-on:
-  push:
-    branches:
-      - main
-  pull_request:
-    branches:
-      - main
-
-jobs:
-  build:
-    runs-on: ubuntu-latest
-    name: "nix build"
-    timeout-minutes: 35
-    steps:
-      - uses: actions/checkout@v3
-        with:
-          # Nix Flakes doesn't work on shallow clones
-          fetch-depth: 0
-      - uses: cachix/install-nix-action@v18
-      - uses: cachix/cachix-action@v12
-        with:
-          name: zellij
-          # If you chose API tokens for write access OR if you have a private cache
-          authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
-      - run: |
-          nix build || nix build --substituters 'https://cache.nixos.org' --extra-substituters ''