software/simpleran: Fix fluentbit ingestion not to barf on large lines

Fluentbit Tail input documentation[1] says that by default maximum buffer
size is 32K which turned out to be too small in practice because we hit
a situation where enb.xlog started to have lines with ~ 34K and so
fluentbit ingestion stopped to work with the following error in
fluentbit log:

    [2024/10/23 20:30:23] [error] [input:tail:tail.0] file=/srv/slapgrid/slappart19/srv/monitor/public/enb.xlog requires a larger buffer size, lines are too long. Skipping file.

-> Fix that by increasing max buffer size to 1M which seems to be high
   enough at least for now.

Maybe it will make sense to configure this as unlimited, but I'm not
sure if going as unlimited is universally a good idea.

[1] https://docs.fluentbit.io/manual/pipeline/inputs/tail

/cc @lu.xu, @jhuge, @tomo
/reviewed-by @paul.graydon
/reviewed-on nexedi/slapos!1672