[PERF] Fix Linux arm64 AOT ROOTFS_DIR missing (Less Impactful)

[LoopedBard3]

This adds the ROOTFS_DIR variable to the global-build-job for Linux AOT Arm64. This is a less impactful and smaller update to fix the problem than #77024 as this does not change global-build-job. Although it may still be worthwhile to check if the crossrootfsDir condition is incorrect and fix it.

Working build with the change: https://dev.azure.com/dnceng/internal/_build/results?buildId=2020245&view=logs&j=29aef6bf-22ff-5269-0c67-e98540f696db&t=d29db927-bdb9-52f6-e00d-5ad3c37b6beb

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Issue Details

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This adds the ROOTFS_DIR variable to the global-build-job for Linux AOT Arm64. This is a less impactful and smaller update to fix the problem than #77024 as this does not change global-build-job. Although it may still be worthwhile to check if the crossrootfsDir condition is incorrect and fix it.

Working build with the change: https://dev.azure.com/dnceng/internal/_build/results?buildId=2020245&view=logs&j=29aef6bf-22ff-5269-0c67-e98540f696db&t=d29db927-bdb9-52f6-e00d-5ad3c37b6beb

Author:	LoopedBard3
Assignees:	LoopedBard3
Labels:	area-Infrastructure-coreclr
Milestone:	-

[LoopedBard3]

This does not have any changes affecting non-perf infra or testing. A run with all impacted spots: https://dev.azure.com/dnceng/internal/_build/results?buildId=2020343&view=logs&j=b883b527-f236-5b5f-eaf1-fa938ac20612.