Extra env vars cause helm to crash

[callumforrester]

The extraEnvVars field in the helm chart causes helm to panic when not empty, for example

extraEnvVars: |
 - name: BEAMLINE
   value: i04

leads to

install.go:222: [debug] Original chart version: ""
install.go:239: [debug] CHART PATH: /home/vid18871/projects/bluesky/blueapi/helm/blueapi


Error: template: blueapi/templates/deployment.yaml:97:27: executing "blueapi/templates/deployment.yaml" at <.Values.extraEnvVars>: can't evaluate field Values in type string
helm.go:84: [debug] template: blueapi/templates/deployment.yaml:97:27: executing "blueapi/templates/deployment.yaml" at <.Values.extraEnvVars>: can't evaluate field Values in type string

[callumforrester]

Appears to be caused by the with .Values.extraEnvVars that was added in #520