Missing `mod` tool for Go module major upgrades (gomodUpdateImportPaths)

[dnwe]

If the user has enabled the option gomodUpdateImportPaths in the postUpdateOptions array, then Renovate tries to use mod to update import paths on major updates. This works fine on the slim image as it spins up a docker container with mod installed. However, it appears that mod is not currently included in the full image:

Command failed: mod upgrade --mod-name=github.com/pierrec/lz4 -t=4\n/bin/sh: 1: mod: not found\n