Speed up travis builds by skipping the .net core initialisation (#868)

Author: bergmeister

.travis.yml

@@ -2,6 +2,10 @@ language: cpp
 
 git:
   depth: 1000
+  
+env:
+  # Avoid expensive initialization of dotnet cli, see: https://donovanbrown.com/post/Stop-wasting-time-during-NET-Core-builds
+  DOTNET_SKIP_FIRST_TIME_EXPERIENCE=1
 
 os:
   - linux