Do not fail PostgreSQL install on arm64

[Joe Bowbeer]

Bitnami postgresql chart now supports arm64:

https://hub.docker.com/r/bitnami/postgresql/tags

Expected Behaviour

arkade install postgresql succeeds on k8s with arm64 nodes

Current Behaviour

arkade install postgresql fails on k8s with arm64 nodes

Possible Solution

N/A

Steps to Reproduce (for bugs)

1. Create cluster on MacBook Pro w/M1 chip
2. arkade install postgresql

[Alex Ellis]

Commented here - #943 (comment)

Thanks for sending this.