Use Helm global.externalIstiod for pilot env variable

[nshankar13]

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Describe the feature request

For multi-cluster primary-remote installation with Helm, externalIstiod needs to be set both for .Values.global and .Values.pilot.env. This PR consolidates the installation setup to just use .Values.global to set the pilot env variable. However, to prevent duplication (setting the same env variable twice), if EXTERNAL_ISTIOD is set via the pilot env variable, then respect that instead.

Describe alternatives you've considered

Affected product area (please put an X in all that apply)

[ ] Ambient
[ ] Docs
[ ] Dual Stack
[X] Installation
[X] Networking
[ ] Performance and Scalability
[ ] Extensions and Telemetry
[ ] Security
[ ] Test and Release
[ ] User Experience
[ ] Developer Infrastructure

Affected features (please put an X in all that apply)

[X] Multi Cluster
[ ] Virtual Machine
[ ] Multi Control Plane

Additional context