[tlbuild] report on building locally on an old macos and a query

[Karl Berry]

    I discover only now that fmtutil-{sys,user} insists on using
    the existing binary in the default TeXLive location, 

Not to my knowledge.

    would be that the tex.fmt and etex.fmt actually do not depend
    on the way tex/pdftex is built ? as long as we are talking of the
    same source and same Plain.tex

Right, the same fmt files should result from the same sources, no matter
how they are compiled (within reason). Barring compiler bugs. -k