fix: prioritize user-specified env vars

[nschmeller]

Currently, user-specified env vars are treated differently between the metaflow-service and metaflow-ui Helm charts.

In the metaflow-service Helm chart, user-specified env vars are placed after chart-specified env vars. In this case, K8s will treat them with priority based on order. However, in the metaflow-ui Helm chart, the chart-specified env vars are placed last, giving them priority. Besides being inconsistent, this is problematic as we want to set our own env vars that conflict with the chart-specified env vars (in this case, MF_METADATA_DB_USER and MF_METADATA_DB_PSWD).

This commit switches the layout of the metaflow-ui env vars to prioritize user-specified over chart-specified env vars.

metaflow-service layout:

metaflow-tools/charts/metaflow/charts/metaflow-service/templates/deployment.yaml

Lines 73 to 75 in d24572c

	env:
	{{- include "metaflow-service.metadatadbEnvVars" . | nindent 12 }}
	{{- with .Values.env }}

metaflow-ui layout:

metaflow-tools/charts/metaflow/charts/metaflow-ui/templates/backend_deployment.yaml

Lines 51 to 75 in d24572c

	env:
	{{- with .Values.uiBackend.env }}
	{{- toYaml . | nindent 12 }}
	{{- end }}
	- name: UI_ENABLED
	value: "1"
	- name: PATH_PREFIX
	value: "/api"
	- name: MF_DATASTORE_ROOT
	value: {{ .Values.uiBackend.metaflowDatastoreSysRootS3 | quote }}
	- name: METAFLOW_DATASTORE_SYSROOT_S3
	value: {{ .Values.uiBackend.metaflowDatastoreSysRootS3 | quote }}
	{{- if .Values.uiBackend.metaflowS3EndpointURL }}
	- name: METAFLOW_S3_ENDPOINT_URL
	value: {{ .Values.uiBackend.metaflowS3EndpointURL | quote }}
	{{- end }}
	- name: LOGLEVEL
	value: "DEBUG"
	- name: METAFLOW_SERVICE_URL
	value: {{ .Values.uiBackend.metaflowServiceURL | quote }}
	- name: METAFLOW_DEFAULT_DATASTORE
	value: {{ .Values.uiBackend.metaflowDefaultDatastore | quote }}
	- name: METAFLOW_DEFAULT_METADATA
	value: "service"
	{{- include "metaflow-ui.metadatadbEnvVars" . | nindent 12 }}