fix(scaleway): Don't override IPv6 routes when IPv4 is not primary

[NoSuchCommand]

Proposed Commit Message

fix(scaleway): Don't override IPv6 routes when IPv4 is not primary

If an instance in _routed-ip_ mode uses both IPv4 **and** IPv6, but the IPv6 is listed (attached) first, the default IPv6 route may be absent from the final network configuration for systems that do not accept Router Advertisements.

This changes fixes the problem by making sure that routes are appended, not overwritten.

Test Steps

1. Create an instance with routed-ip-enabled=true but no IP address
2. Create a routed_ipv6 Flexible IP and attach it to the instance
3. Create a routed_ipv4 Flexible IP and attach it to the instance
4. Start the instance
5. Once fully booted, connect to the instance and ensure it has the expected network configuration, and especially a default IPv6 route

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)

[TheRealFalcon]

The change looks good but the test in failing on CI. Did you forget to add a mock?

[NoSuchCommand]

The change looks good but the test in failing on CI. Did you forget to add a mock?

It should be better now that I pushed a fix.

[TheRealFalcon]

LGTM!