fix: support system NCCL installations

[janbernloehr]

Summary

DeepEP currently assumes NCCL is installed through the PyPI package
nvidia-nccl-cu12. This fails for standard Ubuntu installations where NCCL
is provided externally, for example:

• /usr/include/nccl.h
• /usr/lib/aarch64-linux-gnu/libnccl.so

This change:

• Discovers NCCL through standard multiarch/system library locations.
• Preserves the existing PyPI-package discovery path.
• Links the exact discovered library without overriding the system NCCL version.
• Avoids adding an unnecessary RPATH for system-installed NCCL.
• Uses the same discovery logic for runtime checks and JIT compilation.

HybridEP does not require this change: its NIXL path skips NCCL/DOCA, while its
DOCA path intentionally builds the bundled NCCL submodule components.

Testing (internal)

• Python syntax and whitespace checks passed.
• System NCCL discovery was validated in the Core Models mbridge build.
• DeepEP 01dc3aaac82068020353dce2c302e38153c0bfaa built and installed
  successfully with system NCCL.
• deep_ep._C metadata validation and import deep_ep passed.