smartagent extension: ensure propagation of collectd bundle dir

[rmfitzpatrick]

These changes fix a bug in the SA Extension that leaves* the default collectd config partially populated and doesn't* respect the desired executable location, which is what the agent actually uses.

I've corrected the relevant unit test with the desired value. The integration tests missed this since they set a temporary collectd configDir option, which I'm not able to bypass at this time due to resulting permission errors w/ the default.

[codecov]

Codecov Report

Merging #180 (a0c7c70) into main (89c8812) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #180   +/-   ##
=======================================
  Coverage   88.62%   88.62%           
=======================================
  Files          16       16           
  Lines         958      958           
=======================================
  Hits          849      849           
  Misses         76       76           
  Partials       33       33           

Impacted Files	Coverage Δ
internal/extension/smartagentextension/config.go	89.74% <100.00%> (ø)

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