formatters: fix typo that broke autoconf'd debian-10 cross builds

A missing '=' made the resulting ./configure complain for libnbd:

./configure --hosti686-linux-gnu --enable-gcc-warnings --with-gnutls --with-libxml2 --disable-fuse --disable-ocaml --disable-python --disable-golang

Fixes: 03f46a17 ("formatters: separate build env variable export from docker commands") Signed-off-by: Eric Blake eblake@redhat.com