It’s hard to answer in general, but you probably have one or more packages that were fixed using Pkg.pin() at some point. It will be easier to see what’s going on if you post the output of Pkg.status(), which will show all of your installed packages and their various versions.
You can set environment variables from within Julia using ENV["variablename"] = "variablevalue". You can make that setting permanent for all Julia sessions by adding such a line to a file named .juliarc.jl in your home directory.