fix: use Contains instead of ContainsKey on OrderedDictionary

In PowerShell 7, [ordered]@{} is System.Collections.Specialized.OrderedDictionary,
which exposes Contains(key) but not ContainsKey(). The Push-Digests task
(multi-arch path) failed on the first include-arm64 dispatch with:
"Method invocation failed because [System.Collections.Specialized.OrderedDictionary]
does not contain a method named 'ContainsKey'."

The amd64-only path via Publish-Direct never hit this branch, which is why the
bug lay dormant since the switch to digest-based multi-arch assembly (0bd74f9).

Author: fdcastel

firebird-docker.build.ps1

@@ -466,7 +466,7 @@ task Push-Digests FilteredAssets, {
             $key = "$($asset.version)/$distribution"
 
             # Push once per version+distro (all tags share the same image)
-            if (-not $digests.ContainsKey($key)) {
+            if (-not $digests.Contains($key)) {
                 Write-Build Cyan "----- [$($asset.version) / $distribution / $hostArch → push-by-digest] -----"
 
                 $metadataFile = Join-Path ([System.IO.Path]::GetTempPath()) "metadata-$($asset.version)-$distribution.json"