x11/alacritty: Apply workaround to let SpawnNewInstance work on FreeBSD

On Linux Alacritty looks up /proc/$shell_pid/cwd to open new instances
in the same working directory as the old instance which does not
work with FreeBSD's procfs(5). As a workaround we can for now
fallback to linprocfs(5), often mounted on /compat/linux/proc, until
there is support for KERN_PROC_CWD and struct kinfo_file in the
Rust ecosystem (libc or elsewhere).

PR: 236382
Submitted by: tobik
Approved by: zeising (maintainer timeout, 2 weeks)