fix(gomod): remove token encoding

[Shegox]

Changes:

This PR checks if a given token starts with x-access-token: as the token is constructed in this way to support GitHub App functionality in other places.
If this is the case only the token part after x-access-token: is encoded.

Context:

Fixes #12339

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[rarkins]

In the hosted app we actually do prefix the received token with x-access-token:, so we need to think if this breaks things or if that prefixing had previously broken things.

https://github.com/renovatebot/renovate/search?q=x-access-token&type=code

[renovate-release]

🎉 This PR is included in version 28.11.1 🎉

The release is available on:

• GitHub release
• 28.11.1

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