Reference the new Mono.Unix nuget (#123)

Mono.Posix.NETStandard is being replaced with the new Mono.Unix
package in Xamarin.Android. Xamarin.Android imports the
`MSBuildReferences.projitems` file which referenced the older
package. This commit switches to the new package.

Author: grendello

src/Microsoft.Android.Build.BaseTasks/MSBuildReferences.projitems

@@ -6,7 +6,8 @@
  <PropertyGroup>
  <MSBuildReferenceVersion>15.1.0.0</MSBuildReferenceVersion>
  <MSBuildPackageReferenceVersion>16.5</MSBuildPackageReferenceVersion>
- <LibZipSharpVersion>1.0.24</LibZipSharpVersion>
+ <LibZipSharpVersion>2.0.0-alpha4</LibZipSharpVersion>
+ <MonoUnixVersion>7.0.0-alpha8.21302.6</MonoUnixVersion>
  </PropertyGroup>
 
  <ItemGroupCondition="$([MSBuild]::IsOSPlatform('windows'))" >
@@ -40,7 +41,7 @@
  <PackageReferenceInclude="K4os.Compression.LZ4"Version="1.1.11" />
  <PackageReferenceInclude="Xamarin.Build.AsyncTask"Version="0.3.4"GeneratePathProperty="true" />
  <PackageReferenceInclude="Xamarin.LibZipSharp"Version="$(LibZipSharpVersion)"GeneratePathProperty="true" />
- <PackageReferenceInclude="Mono.Posix.NETStandard"Version="5.20.1-preview"GeneratePathProperty="true"PrivateAssets="all" />
+ <PackageReferenceInclude="Mono.Unix"Version="$(MonoUnixVersion)"GeneratePathProperty="true" />
  <PackageReferenceInclude="System.Buffers"Version="4.5.1" />
  <PackageReferenceInclude="System.Collections.Immutable"Version="1.7.1" />
  <PackageReferenceInclude="System.Numerics.Vectors"Version="4.5.0" />