Fix ROCm dependency discovery after cget/rbuild toolchain setup

[kentqian]

Summary

• Append ROCm prefix paths after project() so they are not overwritten by the cget toolchain.
• Restore rbuild package behavior without requiring users to set CMAKE_PREFIX_PATH manually.

Testing

• Built packages with CMAKE_PREFIX_PATH unset.
• Verified TheRock DEB contents install under /opt/rocm/extras-10.

Changelog Category

Add a CHANGELOG.md entry for any option other than Not Applicable

• • Added: New functionality.
• • Changed: Changes to existing functionality.
• • Removed: Functionality or support that has been removed. (Compared to a previous release)
• • Optimized: Component performance that has been optimized or improved.
• • Resolved Issues: Known issues from a previous version that have been resolved.
• • Not Applicable: This PR is not to be included in the changelog.

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[Copilot]

Pull request overview

This PR adjusts the top-level CMake configuration to make ROCm dependency discovery robust when using cget/rbuild toolchain setups, by ensuring the ROCm prefix paths are appended after project() (so toolchain-provided post-project() includes don’t overwrite them).

Changes:

• Move the list(APPEND CMAKE_PREFIX_PATH ...) for ROCm-related prefixes to after project(migraphx ...).
• Preserve rbuild package usability when CMAKE_PREFIX_PATH is unset by users.

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