Specify exact python on launch

[jhaigh0]

Description of work

A recent change to how workbench launches seem to have broken the linux stand-alone launch.

The problem was that workbench now launches using a a python command, we don't add the stand-alone bin directory to the PATH so when launching it would either use the wrong python or not launch at all.

This pr changes the command to use the specific python binary found within the bin folder.

Further detail of work

To test:

Test stand-alone launchers launch

https://builds.mantidproject.org/job/build_packages_from_branch/1091/

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[jclarkeSTFC]

Can't get the Windows standalone to start

[jhaigh0]

Can't get the Windows standalone to start

Realised I didn't re-run after switching to sys.executable, sorry about that. Have re-run and updated the link now.

[jclarkeSTFC]

Standalone launches on Linux and Windows with the correct sys.executable path.