[dotnet] Fix nuget target framework (#13253)

dotnet/private/nuget_pack.bzl

@@ -13,16 +13,7 @@ def _guess_dotnet_version(label, assembly_info):
     # The dirname will be something like `bazel-out/darwin_arm64-fastbuild-ST-5c013bc87029/bin/dotnet/src/webdriver/bazelout/net5.0`
     # Note that the last segment of the path is the framework we're
     # targeting. Happy days!
-    full_path = assembly_info.libs[0].dirname
-
-    # And that framework is after the constant string `bazelout`
-    index = full_path.index(label.name)
-
-    # The actual start is at the _end_ of the `bazelout` string
-    index += len(label.name)
-
-    to_return = full_path[index:].replace("/", "")
-    return to_return.replace("/", "")
+    return assembly_info.libs[0].dirname.split("/")[::-1][0]
 
 def nuget_pack_impl(ctx):
     nuspec = ctx.actions.declare_file("%s-generated.nuspec" % ctx.label.name)