(chocolateyGH-1949)(build) ensure files unpacked w/unofficial

When building the Chocolatey package without the official strong name
key, it would not properly create the nuget package due to running
`choco unpackself -f` without the additional
`--allow-unofficial-build`. Instead it would throw an error about
needing that passed. Having that additional switch is ignored on
official builds.

Add it to the `nugetPrepare.post.step` file as part of the arguments
to ensure that the nuget package has all of same components that the
officially strong named package would have.

Author: ferventcoder

.build.custom/nugetPrepare.post.step

@@ -35,7 +35,7 @@
   <target name="unpack_files">
     <echo level="Warning" message="Unpacking Choco files." />
     <property name="app.choco" value="${dirs.drop.nuget}${path.separator}chocolatey${path.separator}tools${path.separator}chocolateyInstall${path.separator}choco.exe" />
-    <property name="args.choco" value="unpackself -f" />
+    <property name="args.choco" value="unpackself -f --allow-unofficial-build" />
     <exec program="${app.choco}"
           workingdir="${dirs.drop.nuget}${path.separator}chocolatey${path.separator}tools${path.separator}chocolateyInstall"
           commandline="${args.choco}" if="${platform::is-windows()}" />