Found when test driving #510 on 'releasepreview' - 1.8.17 by installing juliuap in our cluster.

I installed juliaup and julia systemwide (read only) for users. I added the bin folder to their path and set JULIA_DEPOT_PATH to :/systemwide/location, so environments would go into their home folder, but julia installations would be found in the systemwide location.

Now users couldn't use juliaup or julia(launcher), because it would try to place a lockfile in the systemwide location, for which they don't have permission. I've temporarily allowed writing to the lockfile, but ideally we only place the lock when actually writing the config file. Running julialauncher or juliaup status should only require a read of the config file.

Related to #276