Potential to eliminate code in Julia provisioner

[agoose77]

In #1376, @GeorgianaElena and I did some digging and concluded that a bug in julia<1.4.1 was the reason for the addition of a call to resolve() (https://github.com/fonsp/repo2docker/blob/239c4f57f5a40f7241bb4b4746019eb173475ed8/repo2docker/buildpacks/julia/julia_project.py#L180).

At the time of that PR (#879), the julia command was:

julia --project=${REPO_DIR} -e 'using Pkg; Pkg.instantiate(); pkg"precompile"'

Our reading of this is that at the time, instantiate did not precompile, and resolve() call was then added in #879 to address a bug for missing dependencies.

julia --project=${REPO_DIR} -e 'using Pkg; Pkg.instantiate(); Pkg.resolve(); pkg"precompile"'

Later, in #1376, the call to precompile was changed to instantiate(), to eliminate this warning:

julia --project=${REPO_DIR} -e 'using Pkg; Pkg.instantiate(); Pkg.resolve(); Pkg.instantiate()'

It would appear that for newer Julia versions, the call to resolve() is not necessary, leaving us with

julia --project=${REPO_DIR} -e 'using Pkg; Pkg.instantiate(); Pkg.instantiate()'

Furthermore, the double instantiate() seems superfluous.

Therefore, for newer Julia versions, it would seem that we can remove both resolve() and the second instantiate(). However, we are not familiar enough with repo2docker to make a judgement on whether it is better to leave redundant code, or introduce a julia-version-sensitive branch.

Next steps for this are to test a reproducer for the original #879 to ensure that both #879 and #1736 are solved by simply:

julia --project=${REPO_DIR} -e 'using Pkg; Pkg.instantiate();'

And if so, a maintainer should make a decision about whether we should introduce this simplification through a branch dependent upon the Julia version.