fix(skipcpio): ignore broken pipe

fix(skipcpio): ignore broken pipe

If lsinitrd is called from a context in which SIGPIPE is ignored (e.g.,
from a systemd unit with default setting of IgnoreSIGPIPE=), the
following line will result in an error being issued:

bin="$($SKIP "$image" | { read -r -N 6 bin && echo "$bin"; })"

An example error from `kdumpctl start` (which internally just calls
`systemctl start kdump.service`):

kdumpctl[1287]: ERROR: src/skipcpio/skipcpio.c:191:main(): fwrite

A minimal reproducer:

systemd-run -t sh -c '/path/to/skipcpio /path/to/any/file | false'