Ensure tool package store root is a full path.

This commit fixes the tool package store such that it stores a full path
instead of, potentially, a relative path.

This prevents a relative path from inadvertently being passed to NuGet
during the restore and causing it to restore relative to the temp project
directory.

Fixes #8829.

src/dotnet/ToolPackage/ToolPackageStore.cs

@@ -14,7 +14,7 @@ namespace Microsoft.DotNet.ToolPackage
 
         public ToolPackageStore(DirectoryPath root)
         {
-            Root = root;
+            Root = new DirectoryPath(Path.GetFullPath(root.Value));
         }
 
         public DirectoryPath Root { get; private set; }