'sam build' does not prompt when awaiting input for codeartifact login credentials

[lesterpotter]

Description:
With pip configured to search codeartifact, running sam build on the simple Hello World application waits indefinitely without prompting the user to enter codeartifact credentials.

Steps to reproduce the issue:

1. Create a codeartifact domain and repository.
   • Create domain
   • Create repository and select pypi as an external resource
   • Choose "View connection instruction", choose pip, copy the instructions to clipboard
2. On development system, before running copied command, modify the expiration to 15 minutes
   • paste the copied command into a terminal window/shell
   • add '--duration-seconds 900' to the pasted command
   • run the command
3. Wait 15 minutes for credentials to expire.
4. sam init --name demo-hang --runtime python3.8 --dependency-manager pip --app-template hello-world
5. cd demo-hang
6. sam build

Observed result:
$sam build
Building function 'HelloWorldFunction'
Running PythonPipBuilder:ResolveDependencies
<waits here until hit 'Enter'>
Build Failed
Error: PythonPipBuilder:ResolveDependencies - Could not satisfy the requirement: requests
$

1. Command waits for unidentified user input.
2. Press 'Enter' and the command completes with 'Build Failed', etc, with no indication of underlying codeartifact failure.

Expected result:
Expected either an error or a message as to what sam build is waiting for.

Additional information:

1. Running 'sam build --debug' reveals that the hang is waiting for credentials for codeartifact.
2. Work around is to login using "aws codeartifact login ..." and then run "sam build"
3. Work around requires knowing the codeartifact domain (most people will have only one domain)
4. The system being tested has both Python3.7 and 3.8 installed.