[TCLCORE] Installing Tk 9.0 makes tclsh8.6 unable to find Tk

[Marc C. <cul...@gm...>]

When I install Tk 9.0 (on macOS) and then run tclsh8.6 I get this:

% tclsh8.6
% package require Tk
can't find package Tk

If I then install Tk 8.6,  tclsh8.6 will be able to find Tk, but tclsh9.0
claims to not be able to find Tk.

How do I make both tclsh8.6 and tclsh9.0 work on the same system at the
same time?

And, why do I get such an unhelpful error message?  Couldn't it at least
indicate where it is looking for Tk when it fails to find it?

The tclsh8.6 and tclsh9.0 executables are the ones in /usr/local/bin
installed by running
sudo make -C macosx install

- Marc