sg: fix generate command discarding arguments

Created by: jhchabran

The arguments were incorrectly passed with a .Tail() method, whereas it should be .Slice().

This re-enables to give specific packages to sg generate go.

Test plan

~/work/other US devx/fix-sg-generate-args $ go run ./dev/sg generate go enterprise/dev/ci
✅ go generating 1 packages  
ℹ️ go generate enterprise/dev/ci
ℹ️ goimports -w
ℹ️ go mod tidy
✅ (11s)