[xaprepare] Use NuGet binary we download (#3308)

`NuGetRunner` doesn't properly specify full path to the nuget binary
we download as part of the prepare run and, thus, it depends on any
random version of the utility in the user's `$PATH` (if at all) which
may lead to random errors depending on the nuget version.

Fix the issue by passing the full path to the downloaded binary.

Additionally, fix `ToolRunner` to properly add arguments when managed
runtime (`mono`) is used on Unix.  Until now any initial arguments
were passed to **mono** in such cases, instead of to the managed
executable.  Doh.

build-tools/xaprepare/xaprepare/Application/ProcessRunner.cs

@@ -87,6 +87,22 @@ public ProcessRunner (string command, bool ignoreEmptyArguments, params string[]
 				throw new ArgumentException ("must not be null or empty", nameof (command));
 
 			this.command = command;
+			AddArgumentsInternal (ignoreEmptyArguments, arguments);
+		}
+
+		public ProcessRunner AddArguments (params string[] arguments)
+		{
+			return AddArguments (true, arguments);
+		}
+
+		public ProcessRunner AddArguments (bool ignoreEmptyArguments, params string[] arguments)
+		{
+			AddArgumentsInternal (ignoreEmptyArguments, arguments);
+			return this;
+		}
+
+		void AddArgumentsInternal (bool ignoreEmptyArguments, params string[] arguments)
+		{
 			if (arguments == null)
 				return;
 

build-tools/xaprepare/xaprepare/ToolRunners/NuGetRunner.cs

@@ -10,7 +10,7 @@ partial class NuGetRunner : ToolRunner
 		protected override string ToolName                  => "NuGet";
 
 		public NuGetRunner (Context context, Log log = null, string nugetPath = null)
-			: base (context, log, nugetPath)
+			: base (context, log, nugetPath ?? Configurables.Paths.LocalNugetPath)
 		{}
 
 		public async Task<bool> Restore (string solutionFilePath)

build-tools/xaprepare/xaprepare/ToolRunners/ToolRunner.cs

@@ -87,7 +87,7 @@ protected virtual ProcessRunner CreateProcessRunner (params string[] initialPara
 			if (managedRunner != null)
 				managedRunner = Context.OS.Which (managedRunner);
 
-			var runner = new ProcessRunner (managedRunner ?? FullToolPath, initialParams) {
+			var runner = new ProcessRunner (managedRunner ?? FullToolPath) {
 				ProcessTimeout = ProcessTimeout,
 				EchoCmdAndArguments = EchoCmdAndArguments,
 				EchoStandardError = EchoStandardError,
@@ -103,6 +103,7 @@ protected virtual ProcessRunner CreateProcessRunner (params string[] initialPara
 			if (managedRunner != null)
 				runner.AddQuotedArgument (FullToolPath);
 
+			runner.AddArguments (initialParams);
 			return runner;
 		}