Make BUILD_NAME globally configurable

[Xrayez]

Makes it easier to distinguish between official and custom builds by allowing developers to customize BUILD_NAME env var.

I can make it to default to custom_build over official if you like. Of course, nothing prevents someone to use official name, but this will decrease the likelihood of someone compiling the editor and export templates as official, otherwise the only way to distinguish such builds is by using checksums.

Also note that BUILD_NAME can be fetched at run-time via script with Engine.get_version_info().

[akien-mga]

Looks good! Yeah I think it would make sense to make it default to custom_build, I can set official in my own config.sh.

[Xrayez]

Changed to custom_build by default.

[akien-mga]

Thanks!