[llvm] [workflows] Use /mnt as the build directory on Linux (PR #80583)

[via llvm-commits]

llvmbot wrote:

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Failed to cherry-pick: 1a6426067fb33a8a789978f6e229108787a041be

https://github.com/llvm/llvm-project/actions/runs/7788650343

Please manually backport the fix and push it to your github fork.  Once this is done, please create a [pull request](https://github.com/llvm/llvm-project/compare)

https://github.com/llvm/llvm-project/pull/80583