remove dependency to /bin/sh when using AppRun script

[Samueru-sama]

This guarantees that the application will run even if /bin/sh is not present on the host.

• first checks for common shell locations in /bin/sh, /bin/bash, etc

• finally defaults to using PATH if all of those fail.

Note: It is very important that sharun does not read its .env file when doing this, because otherwise the host shell can inherit env variables from the .env that break.

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Tested it working:

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This can be further extended, for example one can have an AppRun.bash and in that case sharun would look for a bash shell only, same with a AppRun.py where sharun would look for a python executable to run it and so on.