ci: fix pipeline failing on duplicate NuGet package push (#269)

Re-runs of the CI workflow on the same commit/tag produce the same
effective NuGet version because NuGet ignores SemVer build metadata
(`+N`) when comparing versions, causing a 409 Conflict on the second
push.

## Changes

- **`--skip-duplicate`** added to `dotnet nuget push` in `ci.yml` —
silently skips already-published versions instead of failing
- **`MINVERBUILDMETADATA` syntax fixed** — changed
`build.$GITHUB_RUN_ATTEMPT` (unexpanded shell variable reference) to
`build.${{ github.run_attempt }}` (valid GitHub Actions expression)

```yaml
# Before
MINVERBUILDMETADATA: build.$GITHUB_RUN_ATTEMPT
run: dotnet nuget push "./artifacts/*.nupkg" --source "github" --api-key ${{ secrets.GITHUB_TOKEN }}

# After
MINVERBUILDMETADATA: build.${{ github.run_attempt }}
run: dotnet nuget push "./artifacts/*.nupkg" --source "github" --api-key ${{ secrets.GITHUB_TOKEN }} --skip-duplicate
```

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.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
     env:
       DOTNET_NOLOGO: true
       DOTNET_SKIP_FIRST_TIME_EXPERIENCE: true
-      MINVERBUILDMETADATA: build.$GITHUB_RUN_ATTEMPT
+      MINVERBUILDMETADATA: build.${{ github.run_attempt }}
 
     steps:
       - name: ☁ Checkout
@@ -50,5 +50,5 @@ jobs:
             "https://nuget.pkg.github.com/${{ github.repository_owner }}/index.json"
 
       - name: 🚀 Publish to GitHub packages
-        run: dotnet nuget push "./artifacts/*.nupkg" --source "github" --api-key ${{ secrets.GITHUB_TOKEN }}
+        run: dotnet nuget push "./artifacts/*.nupkg" --source "github" --api-key ${{ secrets.GITHUB_TOKEN }} --skip-duplicate