Attempt to target correct version of nuget packages so that their /tools/ directories are as expected

Author: jeanbern

.nuget/packages.config

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <packages>
-  <package id="Codecov" version="1.7.2" />
-  <package id="OpenCover" version="4.7.922" />
-  <package id="ReportGenerator" version="4.2.19" />
-  <package id="xunit.runner.console" version="2.4.1"/>
+  <package id="Codecov" version="1.7.2"/>
+  <package id="OpenCover" version="4.7.922"/>
+  <package id="ReportGenerator" version="4.2.19" targetFramework="netcoreapp2.1"/>
+  <package id="xunit.runner.console" version="2.4.1" targetFramework="netcoreapp2.0"/>
 </packages>