Fix broken MAR file generation ( `(( count++ ))` breaks now)

While preparing nightly builds in #26073 (moved) we realized that MAR file generation is broken now. For some reason (( count++ )) in common.sh does not work anymore and breaks the build. We worked around that for the full MAR file generation in commit b135c59f65dba827b61379a4945251e148c43291 on tor-browser-60.0.1esr-8.0-1 but should create a proper patch and figure out what's actually going on.

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