Update the timestamps in the nugets to Now

Workaround for https://bugzilla.xamarin.com/show_bug.cgi?id=40973

`nuget pack` is creating a package with timestamps in future. I am told that
this is a know issue with mono.  We will use this workaround until the mono
fix makes it onto the build bots.

common.cake

@@ -53,6 +53,22 @@ Task ("tmp-nuget").IsDependentOn ("libs").Does (() =>
 	PackNuGets (newList.ToArray ());
 });
 
+
+Task ("nuget").IsDependentOn ("libs").IsDependentOn ("nuget-base").Does (() =>
+{
+	var fixer = new FilePath ("../nuget-mono-fix.sh").MakeAbsolute (Context.Environment);
+
+	foreach (var nuget in buildSpec.NuGets) {
+		foreach (var file in GetFiles ("./output/" + nuget.NuSpec.GetFilenameWithoutExtension () + "*.nupkg")) {
+			StartProcess ("/bin/sh",
+				new ProcessSettings {
+					Arguments = string.Format ("{0} {1}", fixer, file),
+				}
+			);
+		}
+	}
+});
+
 Task ("component").IsDependentOn ("nuget").IsDependentOn ("tmp-nuget").IsDependentOn ("component-base");
 
 FilePath GetCakeToolPath ()

nuget-mono-fix.sh

@@ -0,0 +1,20 @@
+#!/bin/sh
+
+curdir=$(pwd)
+
+rm -rf nuget-mono-fix
+FILES=`unzip -Z1 $1`
+unzip $1 -d nuget-mono-fix
+
+cd nuget-mono-fix
+
+
+for filename in $FILES; do \
+	touch $filename; \
+	zip --delete $1 $filename; \
+	zip --update $1 $filename; \
+done
+
+cd $curdir
+rm -rf nuget-mono-fix
+