fix(install): Force the result to be an array

libexec/scoop-install.ps1

@@ -97,15 +97,15 @@ if ($specific_versions.length -gt 0) {
     $difference = $apps
 }
 
-$specific_versions_paths = $specific_versions | ForEach-Object {
+$specific_versions_paths = @($specific_versions | ForEach-Object {
     $app, $bucket, $version = parse_app $_
     if (installed_manifest $app $version) {
         warn "'$app' ($version) is already installed.`nUse 'scoop update $app$(if ($global) { " --global" })' to install a new version."
         continue
     }
 
     generate_user_manifest $app $bucket $version
-}
+})
 $apps = @(($specific_versions_paths + $difference) | Where-Object { $_ } | Sort-Object -Unique)
 
 # remember which were explictly requested so that we can