feat: add `GO_COBERTURA_FLAGS` env var for `gocover-cobertura`

mentioned in commit 3077f766

mentioned in merge request to-be-continuous/samples/golang-cnb-helm-on-kubernetes!44 (merged)

mentioned in merge request to-be-continuous/samples/golang-cnb-on-kubernetes!47 (merged)

mentioned in merge request to-be-continuous/samples/golang-docker-on-aws!69 (merged)

mentioned in merge request to-be-continuous/samples/golang-on-cloudfoundry!49 (merged)

mentioned in merge request to-be-continuous/tools/vault-secrets-provider!87 (merged)

mentioned in merge request to-be-continuous/tools/tracking!50 (merged)

mentioned in merge request sylva-projects/sylva-elements/sylvactl!25 (merged)