Do not install tails-installer to $PATH

As some command line options for tails-installer are not currently functional, we should avoid installing tails-installer to $PATH and instead treat it as a mere helper for tails-installer-launcher. That is, do not install to /usr/bin.

We could do that via the packaging or upstream. And this will probably be reverted once we fix #8549 (closed)

Feature Branch: 451f:tails/jessie

Parent Task: #8549 (closed)

Related issues

Related to #8861 (closed)
Blocks #8557 (closed)

Original created by @u on 10538 (Redmine)

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