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

[John Purnell]

When installing TeX Live using the unix install-tl script and changing the default TEXDIR path from /usr/local/texlive/2024 to say /usr/local/texlive/2024basic (as when the macOS BasicTeX package is graciously prepared by Dick Koch), as well as making the (documented) appropriate changes to TEXMFSYSVAR and XMFSYSCONFIG, TEXMFLOCAL is changed from /usr/local/texlive/texmf-local to /usr/local/texlive/2024basic/texmf-local. Therefore making TEXMFLOCAL release specific, in conflict with the directory's persistent intention as stated in the TL documentation.

In my testing, this (IMO) inappropriate change to TEXMFLOCAL only occurs when the trailing (YYYY release installation) directory of TEXDIR is changed. When any parent directory is changed the updated TEXMFLOCAL remains release independent.