Still cannot build GUB with stable/2.20 branch

[John Mandereau]

Hi folks,

I already asked almost one month ago if we could we cherry-pick the
following commits from master:

88633ac Issue 5469/2: Add `TEX` environemnt variable for texi2pdf
e757c64 Issue 5469/1: Tweak wrapped lines
002382c Issue 5463: Fix dblatex uses xetex backend

Without them, GUB build on stable/2.20 branch fails on lilypond-test,
because etex is wrongly called. Are there good objections to
backporting to stable/2.20 branch the three commits above, to fix this
known issue?
Now I can build GUB from scratch in only 4 to 5 hours, so I'd like to
test GUB changes on the two branches which are going to be used for the
next releases.

Best,
John