Remove build number from artefact name.

CI already publishes artefacts in folders versioned with the build
number. We don't need the build number in the filename too, and removing
it will produce cleaner URLs.
Cleanup out folder before building new artefact

This ensures there is no published artefact in case the script fails
Script fails at first error
Override location of credential files using env

This way the script does not have to know the configuration details of
the machine it is running on, and it defaults to standard project
locations so that it can be run locally for testing
Merge pull request #18 from alphagov/artefact-naming

Generate artefact with an unversioned name