Fix CI builds

Update actions/setup-dotnet to 1.7.2 to avoid the use of deprecated
`set-env` command, and add Nuget config to avoid NU1011 (solution copied
from codingteam/emulsion#129).

Author: Minoru

.github/workflows/main.yml

@@ -36,7 +36,7 @@ jobs:
 
       # Backend
       - name: Setup .NET Core SDK
-        uses: actions/setup-dotnet@v1.4.0
+        uses: actions/setup-dotnet@v1.7.2
         with:
           dotnet-version: '3.1.201'
       - name: NuGet Cache

NuGet.Config

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="utf-8"?>
+
+<configuration>
+    <packageSources>
+        <clear />
+        <add key="nuget.org" value="https://api.nuget.org/v3/index.json" protocolVersion="3" />
+    </packageSources>
+</configuration>

codingteam.org.ru.sln

@@ -1,4 +1,4 @@
-
+
 Microsoft Visual Studio Solution File, Format Version 12.00
 # Visual Studio 15
 VisualStudioVersion = 15.0.26124.0
@@ -13,6 +13,7 @@ ProjectSection(SolutionItems) = preProject
 	Readme.md = Readme.md
 	Dockerfile = Dockerfile
 	.dockerignore = .dockerignore
+	NuGet.Config = NuGet.Config
 EndProjectSection
 EndProject
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = ".github", ".github", "{339BA2DC-3628-4E8E-98CC-264B5A3625AA}"