freebsd-slave.sh: Append to fstab instead of replacing its contents.
Fixes an issue where the slaves would run, but couldn't start anymore upon a reboot.
@@ -20,7 +20,7 @@ pkg install -y bazel
## procfs and fddescfs
mount -t fdescfs fdesc /dev/fd
mount -t procfs proc /proc
-cat > /etc/fstab <<EOF
+cat >> /etc/fstab <<EOF
fdesc /dev/fd fdescfs rw 0 0
proc /proc procfs rw 0 0
EOF
