Fix extraEnvVars in helm chart (#582)

Fixes #581 

`extraEnvVars` was being treated as a string in some places and a list
in others. This PR makes it a list everywhere, any values files that
look like this should be updated to remove the `|`:

```yaml
extraEnvVars: |
  - name: BEAMLINE
    value: i04
```

helm/blueapi/templates/deployment.yaml

@@ -92,10 +92,8 @@ spec:
             - "-c"
             - "/config/config.yaml"
             - "serve"
-          {{- with .Values.extraEnvVars }}
           env:
-            {{- tpl .Values.extraEnvVars . | nindent 10 }}
-          {{- end }}
+            {{- toYaml .Values.extraEnvVars | nindent 12 }}
       {{- with .Values.nodeSelector }}
       nodeSelector:
         {{- toYaml . | nindent 8 }}