ci: silence setup-soldr workflow annotations (linker default + cache size + prebuild-deps miss) #400

Spun off from #382 (CH32V307 warning cleanup).

The setup-soldr GitHub Action emits warning annotations on every fbuild
workflow run, independent of the underlying board build. Even after #382
silences the CH32V307 compiler warnings, the action-level annotations
remain. Example from the post-#382 CI run on PR #399:

• https://github.com/FastLED/fbuild/actions/runs/26915267711

Annotations observed:

• setup-soldr: defaulting SOLDR_LINKER=fast ... Set linker: platform-default ... or linker: <value> to silence this warning.
• setup-soldr: build-cache cache payload is 1.12GB before compression (>512.0MB).
• soldr cook prebuilt dependencies, but this job's compile cache recorded N miss(es) and 0 hit(s) ... set prebuild-deps: none to skip the unused cook.

These three are all configuration-level signals from the action itself,
not from fbuild's build pipeline. They should either be silenced by
setting the recommended workflow inputs (linker, prebuild-deps, cache
pruning) or accepted explicitly so they no longer count as build noise.

Out of scope for #382 because the fix is in the workflow template /
caller side rather than in the fbuild Rust codebase.