Make Nix GHC 9.2.3 CI build run only manually

There is no cache for it in Hydra, and there is no cache for /nix/store
in this CI setup. It takes almost 1.5 hours to build it.
Will run it manually on demand.

Author: unclechu

.github/workflows/nix-manual.yml

@@ -0,0 +1,31 @@
+name: Nix Manual CI
+
+on:
+  workflow_dispatch:
+    inputs:
+      withHLS:
+        description: 'Build HLS'
+        required: true
+        default: 'false' # HLS fails to build against GHC 9.2.3 at the moment, turning it off.
+
+jobs:
+  nix-build-and-test-ghc-9_2_3:
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macOS-latest]
+
+    runs-on: ${{ matrix.os }}
+
+    steps:
+      - uses: actions/checkout@v2.4.0
+
+      - name: Install Nix
+        uses: cachix/install-nix-action@v15
+        with:
+          extra_nix_config: |
+            access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
+            system-features = nixos-test benchmark big-parallel kvm
+
+      # nix-build includes running tests for the Haskell library too.
+      - run: |
+          nix-build --argstr ghcVersion ghc923 --arg withHLS ${{ github.event.inputs.withHLS }} -o result-ghc-9.2.3

.github/workflows/nix.yml

@@ -26,25 +26,3 @@ jobs:
 
       # nix-build includes running tests for the Haskell library too
       - run: nix-build
-
-  nix-build-and-test-ghc-9_2_3:
-    strategy:
-      matrix:
-        os: [ubuntu-latest, macOS-latest]
-
-    runs-on: ${{ matrix.os }}
-
-    steps:
-      - uses: actions/checkout@v2.4.0
-
-      - name: Install Nix
-        uses: cachix/install-nix-action@v15
-        with:
-          extra_nix_config: |
-            access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
-            system-features = nixos-test benchmark big-parallel kvm
-
-      # nix-build includes running tests for the Haskell library too.
-      # HLS fails to build against GHC 9.2.3 at the moment, turning it off.
-      - run: |
-          nix-build --argstr ghcVersion ghc923 --arg withHLS false -o result-ghc-9.2.3