Update charts-build-base image to Helm 3
Updates charts-build-base image to use Helm 3.4.0. This is expected to be compatible with the existing version of kubectl in use (1.16.4) given the version skew docs: https://helm.sh/docs/topics/version_skew
Related to gitlab-org/charts/gitlab#2350 (closed)
Will likely need to be merged at the same time as gitlab-org/charts/gitlab!1650 (merged)
Edited by Mitchell Nielsen