sandbox: stop mounting new filesystem on /tmp

Mounting new, empty filesystem on /tmp makes impossible to use mat2 for manipulating files stored there. Especially it breaks running tests while creating package and using /tmp as temporary builddir which is common setup in Arch Linux:
https://aur.archlinux.org/packages/mat2/#comment-721221
10 jobs for master in 3 minutes and 28 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Linting
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linting:bandit

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linting:codespell

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linting:mypy

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linting:pyflakes

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linting:pylint

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tests:archlinux

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tests:debian

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tests:debian_with_bubblewrap

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tests:fedora

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tests:gentoo

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