hotfix: spack cherry-pick: acts: pass root variant to examples

[wdconinc]

Briefly, what does this PR introduce? Please link to any relevant presentations or discussions.

This PR ensures that also for Acts v46 we continue to build the ROOT bindings, which moved from plugins to the examples.

What is the urgency of this PR?

• High (please describe reason below)
• Medium
• Low

This is a dependency of the material validation pipeline in the epic repo.

What kind of change does this PR introduce?

• Bug fix (issue: https://github.com/eic/epic/actions/runs/32656533809/job/97236921179?pr=1159)
• New feature (issue #__)
• Optimization (issue #__)
• Updated documentation
• other: __

Please check if any of the following apply

• This PR introduces breaking changes. Please describe changes users need to make below.
• This PR changes default behavior. Please describe changes below.
• AI was used in preparing this PR. Please describe usage below.

[github-actions]

Capybara summary for PR 397

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• rec_dis_10x100_minQ2=1_epic_craterlake_amd64 (Δ: 1) (new artifact) (ref artifact)
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• sim_dis_5x41_minQ2=1_epic_craterlake_amd64_vs_arm64 (Δ: 395)
• sim_dis_5x41_minQ2=1_epic_craterlake_arm64 (Δ: 1) (new artifact) (ref artifact)
• sim_e_1GeV_20GeV_epic_craterlake_amd64 (Δ: 1) (new artifact) (ref artifact)
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• sim_e_1GeV_20GeV_epic_craterlake_arm64 (Δ: 1) (new artifact) (ref artifact)
• sim_pi_1GeV_20GeV_epic_craterlake_amd64 (Δ: 1) (new artifact) (ref artifact)
• sim_pi_1GeV_20GeV_epic_craterlake_amd64_vs_arm64 (Δ: 359)
• sim_pi_1GeV_20GeV_epic_craterlake_arm64 (Δ: 1) (new artifact) (ref artifact)
  ^{_{Last updated 2026-08-23T17:03-04:00 17dd425}}

[wdconinc]

Not sure why this fails to address the issue...

• Cherry pick of spack/spack-packages@ded706e is correctly adding example_cmake_variant("ROOT", "root"), to /opt/spack-packages/repos/spack_repo/builtin/packages/acts/package.py,
• and jfro5sw acts@46.8.1 contains +examples and +root,
• but /opt/software/linux-x86_64_v2/acts-46.8.1-jfro5swprshtq74fxbod2ywh2xwbw3gw/.spack/spack-configure-args.txt doesn't get a -DACTS_BUILD_EXAMPLES_ROOT=True argument.

Unfortunately .spack/spack-build-out.txt.gz is not saved correctly.

This in ghcr.io/eic/eic_ci:unstable-mr-397-nightly. Not sure what's wrong here.

[wdconinc]

Ultimate test command, should pass python import of:

from acts.examples.root import RootMaterialTrackWriter