build: support only fetching or building when running build-toolchain.sh #795
Description
The toolchain build script currently downloads and then immediately builds the packages it needs, this works fine of course when building interactively, but when trying to package it using a package manager such as Nix where builds do not have network access, it would be nice if there was a way to only download or build the packages, but not both.
I can work around this currently by patching the script to insert an exit after everything is downloaded, but this is a bit janky and also still requires passing the second invocation (that doesn't have network access) of the script a toolchain directory with both the archives and extracted directories to properly skip the downloading code.
So what I propose is simply adding an environment variable that you can set to tell the script to only download the tarballs and stop, and another that will make it assume they already exist and not try to download them again.