Do not use certificate on insecure mode

[sarroutbi]

Refactor the get_https_client function to eliminate code duplication
between the secure and insecure client configuration paths.

This change creates a single reqwest::Client::builder instance and
conditionally applies either danger_accept_invalid_certs(true) for
insecure mode or loads and adds the root certificate specified via
--certificate for secure mode.

This correctly avoids reading and processing the certificate file
unnecessarily when running with --insecure, fixing a logical
inconsistency where the certificate was loaded even if ignored.

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.

Project coverage is 63.57%. Comparing base (17a33ae) to head (d554cbd).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
keylime-push-model-agent/src/main.rs	88.88%	1 Missing ⚠️

Additional details and impacted files

Flag	Coverage Δ
e2e-testsuite	63.57% <88.88%> (+0.09%)	⬆️
upstream-unit-tests	63.57% <88.88%> (+0.09%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
keylime-push-model-agent/src/main.rs	86.84% <88.88%> (+10.59%)	⬆️

... and 5 files with indirect coverage changes

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