ci(bluebuild): bump timeout 60 → 360 min (first-time pulls slow)

166 ran 6hr through STEPS 10-14 (cosign keys cp, stage-bins cp,
nushell pull, pre_build.sh) — fuse-overlayfs with 130+ layers makes
each cp/RUN take ~40min on first build. Subsequent builds will be
faster (cached layers).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.github/workflows/build-bluebuild.yml

@@ -34,7 +34,7 @@ jobs:
     # via runner config. Privileged + userns=host + sock pass-through
     # already wired in the runner config (see infra/forgejo/).
     runs-on: nullstone
-    timeout-minutes: 60
+    timeout-minutes: 360
     permissions:
       contents: read
       packages: write