Packaging repo installation failure for Amazon Linux 2 AMI

We had a community user reported failure from Anand Triphati on slack.

https://citus-public.slack.com/archives/C0XRHT1KJ/p1639042852055100

## Original Message

Hi all, I am installing pg_cron on my Postgres 10 running Amazon Linux 2 AMI and I am getting this error

> Detected operating system as amzn/2.
> Unfortunately, your operating system distribution and version are not supported by this script.
>
> Please contact us via https://www.citusdata.com/about/contact_us with any issues.
> While firing the below command:

```console
curl https://install.citusdata.com/community/rpm.sh | sudo bash
```

--------

## Proposed solution

Install the packaging repo for RHEL7 as the Amazon Linux 2 is based on that distro.

```console
curl https://install.citusdata.com/community/rpm.sh | sudo os=rhel dist=7 bash
```