grubconfig: should use /sysroot for the physical root

[HuijingHei]

Get hints from bootc code https://github.com/bootc-dev/bootc/blob/main/crates/lib/src/install.rs#L2017

[gemini-code-assist]

Code Review

This pull request correctly identifies the need to use /sysroot as the physical root, aligning with practices in tools like bootc. However, there are a few issues with the implementation that should be addressed.

I've left specific comments regarding a critical lifetime issue with how rootdir is created, and a high-severity logical issue where rootdir is not used consistently throughout the function.

Additionally, for improved robustness, you might consider adding a fallback mechanism for when /sysroot does not exist, similar to the bootc implementation referenced in the PR description. The current code will fail in environments without a /sysroot directory.