build(nuget): use -DisableParallelProcessing to try to fix hangs

Since I introduced a new version of nuget (required
by some nuget packages; see 9804b65), the build hangs
usually in gitlab-ci (maybe because of some underlying
Mono problem with HTTP requests, that hopefully is fixed
in a new version... maybe fixed by [1]?).

As a workaround, I'm testing now this flag to see if it
improves things. See the docs here:
https://docs.microsoft.com/en-us/nuget/tools/cli-ref-restore

[1] mono/mono@cd09ddb

Author: knocte

NuGet.Restore.targets

@@ -76,12 +76,12 @@
 
 		<Message Importance="$(RestoreLoggingImportance)" Text="Restoring $(_RestoreSolution)..." />
 
-		<Exec Command='$(Mono) "$(_NuGet)" $(_RestoreCommand) "$(_RestoreSolution)" $(_RestoreArgs)'
+		<Exec Command='$(Mono) "$(_NuGet)" $(_RestoreCommand) "$(_RestoreSolution)" -DisableParallelProcessing $(_RestoreArgs)'
 			  ContinueOnError="$(ContinueOnRestoreError)" 
 			  Condition="'$(_RestoreOutput)' == ''"
 			  WorkingDirectory="$(_RestoreDir)" />
 
-		<Exec Command='$(Mono) "$(_NuGet)" $(_RestoreCommand) "$(_RestoreSolution)" -OutputDirectory "$(_RestoreOutput)" $(_RestoreArgs)'
+		<Exec Command='$(Mono) "$(_NuGet)" $(_RestoreCommand) "$(_RestoreSolution)" -DisableParallelProcessing -OutputDirectory "$(_RestoreOutput)" $(_RestoreArgs)'
 			  ContinueOnError="$(ContinueOnRestoreError)" 
 			  Condition="'$(_RestoreOutput)' != ''"
 			  WorkingDirectory="$(_RestoreDir)" />