fix: use the correct base User-Agent for metal-go requests

[ctreatma]

The code that injects terraform module information into the User-Agent string for the metal-go client was mis-wired; it was using the packngo User-Agent as a base instead of the metal-go User-Agent. This change will correct the User-Agent so that requests made with metal-go will not appear to come from packngo.

[codecov-commenter]

Codecov Report

Attention: 29 lines in your changes are missing coverage. Please review.

Comparison is base (26b2f3d) 60.11% compared to head (dcfc5f4) 60.03%.
Report is 31 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #437      +/-   ##
==========================================
- Coverage   60.11%   60.03%   -0.09%     
==========================================
  Files          99       99              
  Lines       20045    20040       -5     
==========================================
- Hits        12051    12031      -20     
- Misses       7691     7704      +13     
- Partials      303      305       +2     

Files	Coverage Δ
equinix/config.go	64.36% <100.00%> (+1.06%)	⬆️
equinix/errors.go	86.95% <100.00%> (+0.59%)	⬆️
equinix/fabric_cloud_router_schema.go	100.00% <100.00%> (ø)
equinix/helpers_device.go	82.26% <97.14%> (-2.65%)	⬇️
equinix/fabric_mapping_helper.go	0.77% <0.00%> (ø)
equinix/data_source_metal_device.go	88.33% <89.47%> (-0.71%)	⬇️
equinix/resource_fabric_cloud_router.go	9.69% <0.00%> (-0.05%)	⬇️
equinix/resource_metal_device.go	78.63% <87.50%> (-0.45%)	⬇️
equinix/resource_fabric_connection.go	5.75% <0.00%> (-0.09%)	⬇️

... and 5 files with indirect coverage changes

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