fix: Only add / for lookup, not for returned registry

[candrews]

Changes:

When checking to see if lookupName starts with the registry, append a / (if not already present) to the registry.
Do not append the / when returning the registry.

Note that the registry URL cannot end in a / as the / is appended to it each time it's used. For example, see

renovate/lib/datasource/docker/index.ts

Line 137 in 7165abd

const apiCheckUrl = `${registry}/v2/`;

Context:

Fixes #8649

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[renovate-release]

🎉 This PR is included in version 24.67.1 🎉

The release is available on:

• GitHub release
• 24.67.1

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