environmentVar could use a "unique" constraints

[jkowalleck]

          since a var can only have exactly one value, it is desireable to have a 

constraint that each var is used only once in the context of it's environmentVars container.

Originally posted by @jkowalleck in https://github.com/CycloneDX/specification/pull/222#discussion_r1200210263

[jkowalleck]

I'd suggest to drop this from 1.6 this looks like a breaking change !