Fix dead link in build.sh help message to point to correct cross-building documentation

[Copilot]

The build.sh --help command contained a dead link that referenced a non-existent file:

However, for this example, you need to already have ROOTFS_DIR set up.
Further information on this can be found here:
https://github.com/dotnet/runtime/blob/main/docs/workflow/building/coreclr/linux-instructions.md

The link pointed to linux-instructions.md which returns a 404 error. This change updates the URL to point to the correct documentation file cross-building.md, which contains the relevant information about setting up ROOTFS_DIR for Linux cross-compilation.

Changes:

• Updated the URL in eng/build.sh line 120 from linux-instructions.md to cross-building.md

The cross-building.md file contains comprehensive documentation about cross-building, including the Linux cross-building section with detailed instructions on generating ROOTFS and setting up the ROOTFS_DIR environment variable.

Fixes #117320.

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[jkotas]

/ba-g Build ios-arm64 Release AllSubsets_Mono timeout