ci: align renovate and crossbuild with authelia/authelia

[nightah]

Return Renovate to its default renovate/ branch prefix. branchPrefix was set to renovate- only because renovate/ could not be used verbatim as a container tag. That constraint never applied here, since this repository publishes no images and no branch name is turned into a tag or file name, and authelia/authelia has since dropped the override in authelia/authelia#13249. branchPrefixOld retires the branches left on the old prefix, and the draft bypass in .buildkite/pipeline.sh matches ^(dependabot|renovate) and so covers both.

Take the Go toolchain, headers, goreleaser, grype and syft mounted into authelia/crossbuild from /buildkite/.tools rather than /usr, and drop the fixed pam-authelia-crossbuild container name, matching authelia/authelia#12726. On a Docker daemon shared between the agents on a host, bind mount sources are resolved against the host rather than inside the agent that requested them, so the agent image's own /usr is not visible there, and a fixed name collides whenever two builds run on the same host at once. The unit-test agents this step targets are the same ones authelia/authelia already builds on with these paths, so /buildkite/.tools is in place on them today.

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• .buildkite/steps/build.sh
• .renovaterc

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