Bump NuGet.CommandLine from 6.8.0 to 6.9.1 (#76)

Bumps [NuGet.CommandLine](https://github.com/NuGet/NuGet.Client) from 6.8.0 to 6.9.1.
- [Release notes](https://github.com/NuGet/NuGet.Client/releases)
- [Commits](https://github.com/NuGet/NuGet.Client/commits)

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

build.cake

@@ -1,4 +1,4 @@
-#tool "nuget:?package=NuGet.CommandLine&version=6.8.0"
+#tool "nuget:?package=NuGet.CommandLine&version=6.9.1"
 
 #addin "nuget:?package=Cake.MinVer&version=2.0.0"
 #addin "nuget:?package=Cake.Args&version=2.0.0"