setup: simplify cache branch

[sshane]

cached installed broken since #35960, silently wasn't running installer to fetch and checkout

on mici before commaai/flash#191, flash.comma.ai was flashing a custom mici userdata with cache that likely had openpilot properly on release-mici (ccf7361)

then we do a fcopyfile in setup which isn't atomic, and comma.sh may not run the installer:

log from comma.sh

running installer                                                                                               
/usr/comma/comma.sh: line 101: /tmp/installer: Text file busy

then it continues to run continue.sh written from setup: https://github.com/commaai/agnos-builder/blob/6655610bc9d95aa42f169c27cae5a4520aedcf05/userspace/usr/comma/comma.sh#L78-L80

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now after that flash PR, we flash userdata on release-tizi (built from https://github.com/commaai/agnos-builder/blob/master/scripts/build_userdata.sh), installer never runs to check out proper branch, and it breaks the install

and on tici, it sometimes silently doesn't fetch using cache

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[sshane]

I kept it in this bad state on release-tizi, then downloaded the release installer from azure, then touched /data/.openpilot_cache and ran the installer, and it used cached fetch and checked out release-mici and comma.sh launched openpilot properly.