fix sdk integration for .net core paket on osx/unix

if exists `.paket/paket`, use that

Author: enricosada

src/Paket.Core/embedded/Paket.Restore.targets

@@ -14,6 +14,7 @@
     <MonoPath Condition="'$(MonoPath)' == '' And Exists('/Library/Frameworks/Mono.framework/Commands/mono')">/Library/Frameworks/Mono.framework/Commands/mono</MonoPath>
     <MonoPath Condition="'$(MonoPath)' == ''">mono</MonoPath>
     <!-- Paket command -->
+    <PaketExePath Condition=" '$(PaketExePath)' == '' AND Exists('$(PaketToolsPath)paket')">$(PaketToolsPath)paket</PaketExePath>
     <PaketExePath Condition=" '$(PaketExePath)' == '' AND Exists('$(PaketRootPath)paket.exe')">$(PaketRootPath)paket.exe</PaketExePath>
     <PaketExePath Condition=" '$(PaketExePath)' == '' ">$(PaketToolsPath)paket.exe</PaketExePath>
     <PaketCommand Condition=" '$(OS)' == 'Windows_NT'">"$(PaketExePath)"</PaketCommand>