[tlbuild] rebuild for mpost/luatex btex fix

[Karl Berry]

    ak> It seems that you forgot to update pmpostdir/pmp.ch.

Thanks. I copied that in and now the branch builds for me.

   js> Only in texlive-20210324-source/utils/asymptote: tr.d

Sorry for that silliness. I suppose I'm crazy, but somehow I am not
enthused about committing another 50mb file just to get rid of
harmless/useless object files left over from my asy test build. The
texlive-20210325-source.tar.xz in historic/systems/texlive/2021 doesn't
have them. (There are no other changes between the two source.tar.xz's.)

I put a line in my notes to help remind to distclean asy next
year. --thanks, karl.