ci(gfx11): add weekly master+gfx11 builds on frozen ROCm 7.14

[jimw567]

Summary

Extends the Build gfx11 + ROCm workflow from a single nightly build to three variants, driven by a new setup matrix job:

Variant	Source ref	ROCm channel	Rolling tag	Cadence
A	gfx11	latest nightly (7.15)	gfx11-rocm-nightly	nightly (unchanged)
B	master	7.14 release (frozen)	master-rocm-714	weekly (Thu)
C	gfx11	7.14 release (frozen)	gfx11-rocm-714	weekly (Thu)

The two new 7.14 variants build weekly (Thursday, ahead of weekend regressions) so we can run regressions against a fixed ROCm 7.14 while the llama.cpp source varies week to week.

Details

• New setup job emits the build matrix from the event: push/PR build only variant A's shape (triggering ref + latest, no release) so PR CI cost/latency is unchanged; workflow_dispatch picks legs via a build_set input (nightly / weekly / all).
• ROCm channel resolution: the detector now accepts a bare X.Y channel (e.g. 7.14) -> highest therock-dist-linux-multiarch-7.14.0a* tarball, alongside latest (highest overall) and an explicit pinned version override. The 7.14 line is frozen at 7.14.0a20260624 (last build before 7.15 alphas began), so "7.14 release" is deterministic.
• build-ubuntu / test-gfx / create-release are all matrixed over the variants. Per-variant ccache keys and artifact names avoid cross-variant thrash/collision; each build writes build-meta.txt (ref, commit, ROCm version) into its artifact so downstream jobs read provenance without fragile matrix outputs.
• Rolling per-variant tags: create-release deletes and recreates each variant's tag at the exact built commit, so each tag always points at the newest build.

Note: variant A's release tag changes from dated b<YYYYMMDD> to the rolling gfx11-rocm-nightly.

Cadence wiring

GitHub cron only fires from the default branch, so the weekly build uses the existing external-cron mechanism (same as the nightly). Add on the CI host:

# weekly Thursday (variants B + C)
7 9 * * 4  gh workflow run build-gfx11-rocm.yml --repo AMD-Ecosystem/llama.cpp --ref gfx11 -f build_set=weekly -f create_release=true

Test plan

• Dispatch build_set=nightly (create_release=false) - confirm 1 leg (A), builds latest ROCm, gfx1151 test passes.
• Dispatch build_set=weekly (create_release=false) - confirm 2 legs (B master, C gfx11), both resolve ROCm 7.14.0a20260624, gfx1151 tests pass.
• Dispatch build_set=weekly create_release=true - confirm rolling tags master-rocm-714 and gfx11-rocm-714 publish at the built commits.
• Confirm push/PR to gfx11 still builds only variant A and never releases.