[SPI-2082] fix Julia DEPOT LOAD PATH usage
@mato may want to look/try this.
This fixes a usage annoyance where if user didn't have
~/.julia/environments/v1.6/Project.toml and try to
Pkg.add stuff, Julia would try to install to the
cvmfs read-only path because that's the only active project Julia sees.
After this fix, Julia can still load from
cvmfs (i.e. SWAN would still work).
: in LOAD_PATH is important because it will expand to the default:
["@", "@v#.#", "@stdlib"]
in Julia. See reference https://docs.julialang.org/en/v1/base/constants/#Base.LOAD_PATH