[bug] revert setting of TMPDIR

setting TMPDIR attempted to end the noisy error message about using TMP
producing non readable messages.

however, this produces invisible icons in bionic.

See riseup_vpn#44
4 jobs for 0.18.10 in 7 minutes and 16 seconds
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passed #78065
linux
build_test

00:04:43

failed #78068
yosemite osx allowed to fail
osx_installer

passed #78067
linux
snap

00:07:15

passed #78066
linux
win_installer

00:02:10

 
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