Fix sporadic warmup binary build failure in CI (#5797)

The warmup binary was being copied from a hardcoded bazel-bin path that
doesn't work reliably with cross-compilation and remote caching. When the
binary was cached but not materialized locally, the cp command would fail.

Fix by:
- Create a warmup_tar pkg_tar target that packages the warmup binary
- Use tar extraction instead of cp, which forces Bazel to materialize
  the output file before the command runs
- Rename the binary from warmup_linux_amd64 to warmup in the tar

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

.github/workflows/docker_build.yml

@@ -68,11 +68,11 @@ jobs:
             //ci:eagle_server_image \
             //ci:admin_server_image \
             //ci:jfr_sidecar_image \
-            //src/main/go/net/eagle0/warmup:warmup_linux_amd64
+            //ci:warmup_tar
 
-          # Copy warmup binary to scripts/ for deployment
+          # Extract warmup binary from tar for deployment
           mkdir -p scripts/bin
-          cp bazel-bin/src/main/go/net/eagle0/warmup/warmup_linux_amd64_/warmup_linux_amd64 scripts/bin/warmup
+          tar -xf bazel-bin/ci/warmup_tar.tar -C scripts/bin --strip-components=1
 
           # Save all image paths before any other bazel command changes bazel-bin symlink
           EAGLE_PATH=$(readlink -f bazel-bin/ci/eagle_server_image)
@@ -201,9 +201,9 @@ jobs:
 
       - name: Build warmup tool
         run: |
-          bazel build //src/main/go/net/eagle0/warmup:warmup_linux_amd64
+          bazel build //ci:warmup_tar
           mkdir -p scripts/bin
-          cp bazel-bin/src/main/go/net/eagle0/warmup/warmup_linux_amd64_/warmup_linux_amd64 scripts/bin/warmup
+          tar -xf bazel-bin/ci/warmup_tar.tar -C scripts/bin --strip-components=1
 
       - name: Copy config files to droplet
         run: |

ci/BUILD.bazel

@@ -1,6 +1,19 @@
 load("@rules_oci//oci:defs.bzl", "oci_image", "oci_load", "oci_push")
 load("@rules_pkg//pkg:tar.bzl", "pkg_tar")
 
+#
+# Deployment artifacts (tools needed on the host, not in containers)
+#
+
+pkg_tar(
+    name = "warmup_tar",
+    srcs = ["//src/main/go/net/eagle0/warmup:warmup_linux_amd64"],
+    package_dir = "bin",
+    remap_paths = {
+        "/warmup_linux_amd64": "/warmup",
+    },
+)
+
 #
 # Shared utilities layer (busybox for nc, wget, etc.)
 #