offline build Doc

[kingstar158]

Are there any guidelines for offline build ？？

[ViggoC]

@kingstar158 What do you mean "offline build", build in an environment with out network?

[Yohahaha]

download deps to local, then set VELOX_DEPS_URL env variables.
for example, offline build gtest:

1. download https://github.com/google/googletest/archive/refs/tags/v1.13.0.tar.gz to /opt/
2. export VELOX_GTEST_URL=/opt/googletest-1.13.0.tar.gz
3. just start build.