tests: Append instead of write to log file
tests: Append instead of write to log file

Log output of the tectonic cli commands are redirected to a log file.
Instead of overwriting that file on each run, append to it. E.g.
`tectonic install` can be repeated twice. Archiving the logs of both
runs is helpful for debugging.

lander2k2/tectonic-installer,derekhiggins/installer,derekhiggins/installer,squat/tectonic-installer,lander2k2/tectonic-installer,cpanato/tectonic-installer,cpanato/tectonic-installer,squat/tectonic-installer,squat/tectonic-installer,squat/tectonic-installer,kalmog/tectonic-installer,cpanato/tectonic-installer,lander2k2/tectonic-installer,kalmog/tectonic-installer,kalmog/tectonic-installer,cpanato/tectonic-installer,lander2k2/tectonic-installer