ci: move release build job to blacksmith-32vcpu-ubuntu-2404

Step 3 of the Blacksmith rollout from CLI-1497. The build job is the
longest serial step on the release critical path (~6m47s on
large-linux-x86), driving 8x bun --compile plus 6x go build. Moving it
to a 32 vCPU Blacksmith runner should shave 1.5-2 minutes; the
sticky-disk node_modules and go-build caches from the previous steps
compound on top.

Supersedes the org-provisioned large-linux-x86 swap -- per the parent
plan, that label was the only remaining holdout once everything else
migrates to Blacksmith.

Refs CLI-1500.

Author: claude

.github/workflows/release-shared.yml

@@ -45,7 +45,7 @@ on:
 
 jobs:
   build:
-    runs-on: large-linux-x86
+    runs-on: blacksmith-32vcpu-ubuntu-2404
     env:
       BUN_SHELL: ${{ inputs.shell }}
       VERSION: ${{ inputs.version }}