Skip first argument when calling main in launch script

[jclarkeSTFC]

The first argument of sys.argv is the launch script path, so we don't need that.

Fixes #38654

To test:

Check that Windows and macOS installs launch without opening the launch script or giving any warning about it. Also check other arguments, e.g. --single-process. Linux launching is unaffected by this change.

https://builds.mantidproject.org/job/build_packages_from_branch/974/

I've uploaded the conda files to my channel, jclarkestfc/label/launch-script-fix

This does not require release notes because it's a fix for something that hasn't been released yet

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[thomashampson]

I have installed the Conda package on Windows and macOS and can confirm that this works as expected without opening the workbench script or outputting any error message about trying to open the workbench script. It's also allowing me to pass a script to the workbench command to open it in workbench. The --single-process argument also works as expected.