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'

src/skipcpio/skipcpio.c

@@ -23,6 +23,7 @@
 #define _GNU_SOURCE
 #endif
 
+#include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -187,8 +188,10 @@ int main(int argc, char **argv)
                         goto end;
                 }
 
+                errno = 0;
                 if (fwrite(buf.copy_buffer, 1, s, stdout) != s) {
-                        pr_err("fwrite\n");
+                        if (errno != EPIPE)
+                                pr_err("fwrite\n");
                         goto end;
                 }
         }