security/netbird: fix build on armv7

A type error in some dependency.
Also replace a really dodgy fake slice whose size overflowed a 32-bit
integer with a proper use of unsafe.Slice().

i386 still fails for an unrelated reason.

Approved by: portmgr (build fix blanket)
MFH: 2025Q3

(cherry picked from commit 44d4a7c01eeaad23768ad3e2b34ba152636327c8)