For testing purposes I generated a 'luatex-plain' format with
```
mtxrun --generate
mtxrun --script plain --make
```
However, using it as
```
PDFTEX="luatex --fmt=luatex-plain" texi2pdf foo.texi
```
fails with
```
texi2dvi: TeX neither supports -recorder nor outputs \openout lines in its log
file
```
Running the `openout.tex` test manually, i.e., calling
```
luatex --fmt=luatex-plain --file-line-error </dev/null '\input' ./openout.tex
```
works just fine.
It seems that the `PDFTEX` environment variable doesn't accept
commands with arguments.
Werner
