Global variables should not shadow options

[dra27]

opam var --global jobs=42 sets the global variable jobs which creates a strange state for opam in that the jobs variable is now 42 but opam will only schedule up to the previous value of jobs.

The correct course of action here is opam option jobs=42 - opam var should not permit shadowing the options.

[dra27]

(this is a "maybe" for 2.2, at least for me...)