Custom toolchains cannot be uninstalled/unlinked

[pmarcelll]

This line checks whether a toolchain exists by looking up the directory, but custom toolchains use symlinks so this check fails.

[jonas-schievink]

This is pretty annoying, the only way to uninstall custom toolchains is to remove the symlink manually

[pmarcelll]

I just noticed in the changelog that it was fixed by #1201.