tests/docker: Install flex in debian-tricore-cross

When flex is not available, binutils sources default to the
'missing' script, but the current script available is not in
the format expected by the 'configure' script:

  $ ./configure
  ...
  /usr/src/binutils/missing: Unknown `--run' option
  Try `/usr/src/binutils/missing --help' for more information
  configure: WARNING: `missing' script is too old or missing
  ...
  checking for bison... bison -y
  checking for flex... no
  checking for lex... no
  checking for flex... /usr/src/binutils/missing flex

  $ make
  ...
  updating ldgram.h
  gcc -DHAVE_CONFIG_H -I. -I. -I. -D_GNU_SOURCE -I. -I. -I../bfd -I./../bfd -I./../include -I./../intl -I../intl  -w -DLOCALEDIR="\"/usr/local/share/locale\""   -W -Wall -Wstrict-prototypes -Wmissing-prototypes -w -c `test -f 'ldgram.c' || echo './'`ldgram.c
  `test -f ldlex.l || echo './'`ldlex.l
  /bin/sh: 1: ldlex.l: not found
  make[3]: *** [Makefile:662: ldlex.c] Error 127
  make[3]: Leaving directory '/usr/src/binutils/ld'
  make[2]: *** [Makefile:799: all-recursive] Error 1

By pass the 'missing' script use by directly installing 'flex'
in the container.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230112155643.7408-1-philmd@linaro.org>
Reviewed-by: Bastian-Koppelmann <kbastian@mail.uni-paderborn.de>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230124180127.1881110-14-alex.bennee@linaro.org>

tests/docker/dockerfiles/debian-tricore-cross.docker

@@ -20,6 +20,7 @@ RUN apt update && \
        bzip2 \
        ca-certificates \
        ccache \
+       flex \
        g++ \
        gcc \
        git \