Update GitLab CI configuration for NuGet deployments

* NuGet deployments are only done for tagged builds now
* Fix unnecessary using statement
6 jobs for master in 2 minutes and 15 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #57
build

00:01:01

canceled #54
build

00:05:14

 
  Test
passed #58
test

00:00:36

canceled #55
test

 
  Pack
passed #59
pack

00:00:36

canceled #56
pack