use nuget 5.9.1 for azure pipelines

[v-pegao]

#7570

With some test, the last worked version docfx.2.58 was packed with nuget.5.9.1. All version later than 5.9.1 are works differently with 5.9.0 till now.
See list of nuget available version on azure pipeline: https://dist.nuget.org/tools.json

[yufeih]

Paste screenshot of the nuget package layout as proof.

[v-pegao]

@yufeih Here is the nuget package layout with the code change(nuget version: 5.9.1):

You can also check the nupkg by downloading the it from this link