Minor fixes to agent installation

[StevenEmelander]

Issue #, if available:
N/A
Description of changes:

• Updated both the install script and the systemd service to use the executable name that is built (aws_secretsmanager_agent) instead of the previous aws-secrets-manager-agent.
• Due to install script running as root, files in bin directory are not owned by the AGENTUSER. This is necessary for systemd run processes. Added additional chown to recursively set the owner to AGENTUSER within AGENTDIR.
• Added minor change to not fail if the AGENTUSER already exists.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.89%. Comparing base (9944b7a) to head (6a5d885).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #24      +/-   ##
==========================================
- Coverage   68.96%   68.89%   -0.07%     
==========================================
  Files          14       14              
  Lines        2887     2887              
  Branches       10       10              
==========================================
- Hits         1991     1989       -2     
- Misses        896      898       +2     

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