Pass FDO_CI_CONCURRENT to DISTRO_EXEC
Different runners have concurrency tuned for different -j amounts, and the official fd.o runners have FDO_CI_CONCURRENT set in their environment so that jobs can use it to tune how many processes they spawn. It would be great if we passed this through to
DISTRO_EXEC so that we could respect that setting within our container builds.
See mesa/mesa!5669 (merged) for use of the flag.