fix(fxa): Use provider for name mapping in linked account email

[nshirley]

Because:

• We're passing the users name to the provider_name mapping
• Which causes the template to display incorrectly

This Commit:

• Uses the correct item for the map

Closes: FXA-13093

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• If applicable, I have modified or added tests which pass locally.
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• I have verified that my changes render correctly in RTL (if appropriate).

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