write out BUILD_VERSION file with version string generated in build

This will ensure the version released to Omnitruck matches alternate
build_version formats (like SemVer).
look for BUILD_VERSION file if `version` option is unset
Merge branch 'OC-5528/release-version-fix'
chef-server-ctl: unset ENV vars that affect Ruby environment
    
Ensure the calling environment (ಠ_ಠ Bundler) does not infect our Ruby
environment if `chef-server-ctl` is called from a Ruby script.