Prevent that lscpu | grep Flags fails on systems that don't report any flags

[bedroge]

The bot/build.sh suddenly failed on a RISC-V node. This node doesn't have any Flags field in the lscpu output:

$ lscpu 
Architecture:          riscv64
  Byte Order:          Little Endian
CPU(s):                4
  On-line CPU(s) list: 0-3

And this causes the grep command introduced in #123 to return 1, and in turn that makes the entire script fail (because of set -e). By adding || echo "") it will just return an empty string in this case.