install-tl: Changing trailing directory of TEXDIR makes TEXMFLOCAL release specific

[Karl Berry]

Hi Dick,

    change the install directory to
    2024basic. I change TEXMFVAR and TEXMFCONFIG to
    ~/Library/texlive/2024basic/texmf-var and
    ~/Library/texlive/2024basic/texmf-config

I think it would be sensible to also (as of next year) change TEXMFLOCAL
to the version-independent /usr/local/texlive/texmf-local, then.
Admittedly it is clearly not a common issue, but I see nothing against
it.  Feels like it would only help.  Wdyt?  --thanks, karl.