Do external package manager check only at startup

[swiatekm]

What does this PR do?

We were doing the check for external package manager use when validating vars for the platform, which happens on every configuration reload. I've made this an attribute of the platform instead, and it's now only checked on startup.

Why is it important?

Reading a bunch of files every time configuration is reloaded is an unnecessary expense.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool

Related issues

• Relates Elastic agent uses too much memory per Pod in k8s #5835 Memory Consumption Issue with Elastic Agent on Kubernetes with high number of resources #5991

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube