Fix eic-spack-mirror source mirror being dropped during package installation

[SebastianPaucar]

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

This PR adds eic-spack-mirror to mirrors.yaml.in so it's present in the generated file (mirrors.yaml) that gets mounted in containers/eic/Dockerfile, and survives into the actual spack install. No SSL errors.

This fixes dawn/dawncut (and any other package relying on eic-spack-mirror) failing to fetch during spack install in CI.

Verified in #388: eic_xl now build successfully on amd64, amd64_v3, and arm64 with dawn/dawncut in xl/spack.yaml.

What is the urgency of this PR?

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

What kind of change does this PR introduce?

• Bug fix (issue #__)
• 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.

[wdconinc]

Ah, of course, yes. Thanks a lot for digging into this!

[github-actions]

Capybara summary for PR 410

• rec_dis_10x100_minQ2=1000_epic_craterlake_amd64 (Δ: 1) (new artifact) (ref artifact)
• rec_dis_10x100_minQ2=1_epic_craterlake_amd64 (Δ: 1) (new artifact) (ref artifact)
• rec_dis_18x275_minQ2=1000_epic_craterlake_amd64 (Δ: 1) (new artifact) (ref artifact)
• rec_dis_18x275_minQ2=1_epic_craterlake_amd64 (Δ: 1) (new artifact) (ref artifact)
• rec_dis_5x41_minQ2=1_epic_craterlake_amd64 (Δ: 1) (new artifact) (ref artifact)
• rec_dis_5x41_minQ2=1_epic_craterlake_amd64_vs_arm64 (Δ: 4219)
• rec_dis_5x41_minQ2=1_epic_craterlake_arm64 (Δ: 1) (new artifact) (ref artifact)
• rec_e_1GeV_20GeV_epic_craterlake_amd64 (Δ: 1) (new artifact) (ref artifact)
• rec_e_1GeV_20GeV_epic_craterlake_amd64_vs_arm64 (Δ: 3506)
• rec_e_1GeV_20GeV_epic_craterlake_arm64 (Δ: 1) (new artifact) (ref artifact)
• rec_pi_1GeV_20GeV_epic_craterlake_amd64 (Δ: 1) (new artifact) (ref artifact)
• rec_pi_1GeV_20GeV_epic_craterlake_amd64_vs_arm64 (Δ: 3456)
• rec_pi_1GeV_20GeV_epic_craterlake_arm64 (Δ: 1) (new artifact) (ref artifact)
• sim_dis_10x100_minQ2=1000_epic_craterlake_amd64 (Δ: 1) (new artifact) (ref artifact)
• sim_dis_10x100_minQ2=1_epic_craterlake_amd64 (Δ: 1) (new artifact) (ref artifact)
• sim_dis_18x275_minQ2=1000_epic_craterlake_amd64 (Δ: 1) (new artifact) (ref artifact)
• sim_dis_18x275_minQ2=1_epic_craterlake_amd64 (Δ: 1) (new artifact) (ref artifact)
• sim_dis_5x41_minQ2=1_epic_craterlake_amd64 (Δ: 1) (new artifact) (ref artifact)
• 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)
• sim_e_1GeV_20GeV_epic_craterlake_amd64_vs_arm64 (Δ: 359)
• 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: 346)
• sim_pi_1GeV_20GeV_epic_craterlake_arm64 (Δ: 1) (new artifact) (ref artifact)
  ^{_{Last updated 2026-09-06T15:42-04:00 22e88dd}}

[SebastianPaucar]

Ah, of course, yes. Thanks a lot for digging into this!

Glad I could help!