vscode: fix extensions dir path

[ameszy]

Description

On macOS, vscode-insiders (via pkgs.vscode.override { isInsiders = true; }) reads extension configuration from (and write to) .vscode/extensions/extensions.json, not under the .vscode-insiders folder. This commit affects only macOS since the author doesn't know and has no device to test the behaviour on other platform like linux.

Checklist

• Change is backwards compatible.

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• Code tested through nix-shell --pure tests -A run.all or nix develop --ignore-environment .#all using Flakes.

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