fix: skip missing mobile bitrise files in stable-sync

[chloeYue]

Problem

After MetaMask/metamask-mobile#30204 (chore: Remove all bitrise artifacts) deleted bitrise.yml from the mobile repo, every run of the Stable Branch Sync workflow now fails on the mobile repo with:

Executing mobile-specific commands...
Error: Command failed: git checkout origin/main -- bitrise.yml
error: pathspec 'bitrise.yml' did not match any file(s) known to git

Solution is to drop the now-stale bitrise.yml entry from the preserve list.

Behavior for other repos (REPO !== 'mobile') is unchanged.

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Note

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[Cal-L]

LGTM