packaging: blindly factor out trap's cleanup function in builddeb
This commit blindly extracts builddeb's trap routine in a dedicated function.
While doing so, I think two bugs are exposed, which will be addressed in the
next commits:
- single quoting around '$CLEANUP' will always evaluate to the literal
'$CLEANUP' regardless of the variable's value. The "if" will always be true.
- the removal operation will not expand $PWD (and a variable expansion would
need double quotes, anyways.
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