How to disable 'mktexpk' in dvipdfmx?

Consider this ex-14-5.mf:

mode:=localfont;
mode_setup;

beginchar("e",10pt#,7.5pt#,2.5pt#);
pickup pencircle scaled (.4pt+blacker);
draw fullcircle scaled .2w shifted (w/2,(h+d)/2-d);
draw fullcircle scaled .6w shifted (w/2,(h+d)/2-d);
draw fullcircle scaled w shifted (w/2,(h+d)/2-d);
endchar;

end

Run metafont on it. All is good:

$ mf ex-14-5
This is METAFONT, Version 2.7182818 (TeX Live 2017) (preloaded base=mf)
(ex-14-5.mf [101] )
Font metrics written on ex-14-5.tfm.
Output written on ex-14-5.600gf (1 character, 696 bytes).
Transcript written on ex-14-5.log.

Now run tex on this file. All is good:

fontx=./ex-14-5 x e bye

$ tex ex-14-5
This is TeX, Version 3.14159265
(ex-14-5.tex [1] )
Output written on ex-14-5.dvi (1 page, 260 bytes).
Transcript written on ex-14-5.log.

When running dvipdfmx ex-14-5 it tries to create the pk file automatically:

ex-14-5 -> ex-14-5.pdf
[1
kpathsea: Running mktexpk --mfmode / --bdpi 600 --mag +0/600 --dpi 600 ex-14-5
<various errors follow>