aws: change AWS_DEFAULT_PROFILE to AWS_PROFILE

The environment variable name used to be AWS_DEFAULT_PROFILE but the
CLI documentation now only mentions AWS_PROFILE.

https://docs.aws.amazon.com/cli/latest/userguide/cli-environment.html

It seems like the CLI was the only tool using AWS_DEFAULT_PROFILE, and
all the AWS SDKs used AWS_PROFILE, so they standardized on it.

https://onetechnical.wordpress.com/2016/10/07/the-curious-case-of-aws_default_profile/

Note: still left AWS_DEFAULT_PROFILE on the method to set the profile to
maintain backwards compatibility.

Close #7354

plugins/aws/aws.plugin.zsh

@@ -24,7 +24,7 @@ _awscli-homebrew-installed() {
 export AWS_HOME=~/.aws
 
 function agp {
-  echo $AWS_DEFAULT_PROFILE
+  echo $AWS_PROFILE
 }
 
 function asp {
@@ -33,7 +33,7 @@ function asp {
   export AWS_DEFAULT_PROFILE=$1
   export AWS_PROFILE=$1
 
-  export RPROMPT="<aws:$AWS_DEFAULT_PROFILE>$rprompt"
+  export RPROMPT="<aws:$AWS_PROFILE>$rprompt"
 }
 
 function aws_profiles {