fix: upload nix binary for macOS (#1327)

Solution:
- add a workflow to upload cache for macos

Co-authored-by: Federico Kunze Küllmer <31522760+fedekunze@users.noreply.github.com>

.github/workflows/test.yml

@@ -132,6 +132,23 @@ jobs:
           path: debug_files.tar.gz
           if-no-files-found: ignore
 
+  upload-cache:
+    if: github.event_name == 'push'
+    needs: ["integration_tests"]
+    strategy:
+      matrix:
+        os: [macos-latest]
+    runs-on: ${{ matrix.os }}
+    steps:
+      - uses: actions/checkout@v2
+      - uses: cachix/install-nix-action@v15
+      - uses: cachix/cachix-action@v10
+        with:
+          name: ethermint
+          signingKey: "${{ secrets.CACHIX_SIGNING_KEY }}"
+      - name: 'instantiate integration test env'
+        run: nix-store -r $(nix-instantiate tests/integration_tests/shell.nix)
+
   test-sim-nondeterminism:
     runs-on: ubuntu-latest
     timeout-minutes: 25