Added "--update" parameter

[joseeden]

Issue #, if available:
Script ran into an issue when it finds that AWS CLI is pre-installed onto the EC2 instance. Adding the "--update" parameter solves this.

Description of changes:
Adding the "--update" parameter to AWS CLI installation step.

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[cartermckinnon]

LGTM.

If there's no existing install:

> sudo ./aws/install --update
You can now run: /usr/local/bin/aws --version

If there's an existing install of the same version:

> sudo ./aws/install --update
Found same AWS CLI version: /usr/local/aws-cli/v2/2.11.0. Skipping install.
> echo $?
0

If there's an existing install of a different version (newer or older):

> sudo ./aws/install --update
You can now run: /usr/local/bin/aws --version