make: fix benchmark target package selection

[Retr0-XD]

Summary

Fixes #12973 by making the benchmark Make target run benchmarks across containerd packages instead of invoking go test without package arguments.

What changed

• Makefile benchmark target now uses:
  • -run ^$ to skip normal tests while running benchmarks
  • ${PACKAGES} so go test gets explicit package arguments
• removed -test.root, which caused root-package handling issues and immediate failure from repo root.

Reproduction

Before this change:

• make benchmark failed immediately from repo root with:

  • .

  • no Go files in .../containerd

After this change:

• make -n benchmark expands to go test ... ${PACKAGES}
• make benchmark starts running package benchmarks instead of failing immediately.

Testing

• make -n benchmark
• make benchmark (started package benchmark execution; long-running suite interrupted locally)

[samuelkarp]

make benchmark (started package benchmark execution; long-running suite interrupted locally)

It looks like this hasn't been tested.

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