Set up NuGet trusted publishing. (#584)

* Set up NuGet trusted publishing.

* Use environment variable for NuGet feed.

Author: teo-tsirpanis

.github/workflows/release.yml

@@ -7,7 +7,7 @@ on:
 jobs:
   Run-Tests:
     uses: ./.github/workflows/tiledb-csharp.yml
-  Release:
+  Pack:
     runs-on: ubuntu-latest
     needs: Run-Tests
     steps:
@@ -23,9 +23,31 @@ jobs:
         uses: actions/upload-artifact@v4
         with:
           name: nuget-release
-          path: pack/
+          path: ./pack
+  Release:
+    runs-on: ubuntu-latest
+    needs: Pack
+    environment: release
+    permissions:
+      id-token: write
+    steps:
+      - uses: actions/checkout@v5
+      - uses: actions/setup-dotnet@v5
+        with:
+          dotnet-version: 8.0.4xx
+      - name: Obtain NuGet API token
+        uses: nuget/login@v1
+        id: nuget-login
+        with:
+          user: ${{ vars.NUGET_USER }}
+        if: vars.NUGET_USER != ''
+      - name: Download package artifacts
+        uses: actions/download-artifact@v4
+        with:
+          name: nuget-release
+          path: ./pack
       - name: Push packages to NuGet
         shell: bash
         run: |
           cd pack
-          dotnet nuget push "*.nupkg" -s https://api.nuget.org/v3/index.json -k ${{ secrets.TILEDB_CSHARP_NUGET_KEY }}
+          dotnet nuget push "*.nupkg" -s ${{ vars.NUGET_FEED }} -k ${{ steps.nuget-login.outputs.NUGET_API_KEY || secrets.NUGET_KEY }}