Set CI B2_CI_VERSION=0

[sdarwin]

Hi,

Going back some years, boostorg/boost-ci provided a new way to configure B2 variables, which takes effect using B2_CI_VERSION=1. For example:

(new) 'B2_LINKFLAGS': '-fuse-ld=gold'

instead of

(previous) 'B2_LINKFLAGS': 'linkflags=-fuse-ld=gold'

Now that most boost libraries have migrated, we'd like to standardize boost-ci onto B2_CI_VERSION=1.

You can opt-out by configuring B2_CI_VERSION=0 and then upgrade later.

This pull request sets B2_CI_VERSION=0, so CI will continue to work, using the previous method.

[sdarwin]

Hi,
It's been almost 2 months since opening this PR. After another month, upstream boost-ci will proceed to switch the default setting: B2_CI_VERSION=1. Please merge this PR to avoid broken CI tests. Thanks.