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Bump Jenkins Go tool to 1.26.8

Matches the Docker builder and the CI workflows, both of which are on
1.26.8. The previous value, 1.25.5, could not build this module at all
since go.mod requires go 1.26.0.

This does not change pipeline behaviour today: nothing in the repo uses
the provisioned toolchain. `make build` shells out to `docker build`, so
compilation happens inside golang:1.26.8-alpine3.24; `make test` is
commented out; and `make list-of-images` and `make clean` are an echo and
an rm. The tools block, GOPATH and checkoutToSubdirectory are left over
from when the pipeline ran go directly.

Note the value is a Jenkins tool-installation name, not a version string,
so "Go 1.26.8" has to exist in the Jenkins global tool configuration or
the pipeline fails at startup.