This PR updates the "tahoe" module's select_executable() logic to always use the bundled Tahoe-LAFS executable in the event that the user is running a frozen/binary build, thereby preventing problems stemming from another, potentially broken or outdated Tahoe-LAFS installation on the user's PATH interfering with the operation of the application. As a bonus, the application startup time should be shortened by a second or two (as a result of no longer having to probe the other tahoe instances on the PATH).

Users still wishing to use their own tahoe installs with Gridsync can still do so by running the application from source or in a python virtual environment and/or by replacing the PyInstaller bundle in the Tahoe-LAFS directory with one their own.