Remove DOTNET_SKIP_FIRST_TIME_EXPERIENCE

It's no longer supported in .NET Core 3.0 and later.

Author: 0xced

.github/workflows/continuous-integration.yml

@@ -7,7 +7,6 @@ env:
   ContinuousIntegrationBuild: true
   DOTNET_CLI_TELEMETRY_OPTOUT: true
   DOTNET_NOLOGO: true
-  DOTNET_SKIP_FIRST_TIME_EXPERIENCE: true
   DOTNET_SYSTEM_CONSOLE_ALLOW_ANSI_COLOR_REDIRECTION: true
   TERM: xterm