Database Download Trait does not respect AWS credentials in environment variables

[apotek]

Description

The Database Download Trait uses the AsyncAWS library to do S3 database backup downloads.

According to the library, AsyncAWS respects environment variables set, for authentication.

But Usher does not allow for this and forces use of credentials stored in $HOME/.aws/credentials

usher/src/Robo/Plugin/Traits/DatabaseDownloadTrait.php

Lines 35 to 42 in eed0a39

	$awsConfigDirPath = getenv('HOME') . '/.aws';
	$awsConfigFilePath = "$awsConfigDirPath/credentials";
	if (!is_dir($awsConfigDirPath) || !file_exists($awsConfigFilePath)) {
	$result = $this->configureAwsCredentials($awsConfigDirPath, $awsConfigFilePath);
	if ($result->wasCancelled()) {
	return Result::cancelled();
	}
	}

According to their documentation, the following sources, in order, should be consulted for credential configuration:

1. Hard-Coded Configuration
2. Environment Variables
3. WebIdentity
4. Credential and Configuration Files
5. ECS Container Credentials
6. EC2 Instance Metadata

In a perfect world, we should support all of these, in this order. Perhaps the perfect world is achievable and Usher just gets out of the way, but if not, we can at least, minimally, allow for 1, 2, and 4, in that order.

This means that, before we run lines 35-42 of the above quoted code, we should:

1. Check to see if the client is already configured with authentication values.
2. Check to see if the required environment variables are set, and
3. Only if the above two conditions are false, continue to our current logic.

[apotek]

Tested all tugboat commands in Tugboat and they work. https://github.com/ChromaticHQ/benz-platform/pull/6220