[BUG] mktextfm requires writable cache

[Henri Menke]

Dear list,

If the TeX Live cache is not writable, it is impossible to generate
fonts.  A non-writable cache is common in sandboxed or otherwise
restricted environments, like a CI service.  The key point here is that
the generation fails, even if an absolute path is specified:

    [homeless at arch tmp]$ mktextfm --destdir /tmp lhr10
    mktextfm: Running mf-nowin -progname=mf \mode:=ljfour; mag:=1; ; nonstopmode; input lhr10
    This is METAFONT, Version 2.7182818 (TeX Live 2020) (preloaded base=mf)
    
    
    kpathsea: Running mktexmf lhr10
    mkdir: cannot create directory '././home/homeless': Permission denied
    ! I can't find file `lhr10'.
    <*> ...=ljfour; mag:=1; ; nonstopmode; input lhr10
                                                      
    Please type another input file name
    ! Emergency stop.
    <*> ...=ljfour; mag:=1; ; nonstopmode; input lhr10
                                                      
    Transcript written on mfput.log.
    grep: lhr10.log: No such file or directory
    mktextfm: `mf-nowin -progname=mf \mode:=ljfour; mag:=1; ; nonstopmode; input lhr10' failed to make lhr10.tfm.

This strikes me as rather odd, because I would have expected that the
--destdir option is designed to work around exactly that situation.  To
reproduce, make sure that your TeX Live installation is not writable and
run mktextfm as a user that doesn't have a home directory.

Cheers,
Henri