Add OVERRIDE_LBEXEC_APP environment variable

When using a project with lb-dev the value of ${GAUDIAPPNAME} is ${PROJECT_NAME}Dev (to avoid circular dependnecies as lb-dev creates a new project).

As lbexec relies on GAUDIAPPNAME to discover how to parse the options.yaml file it's fundimentally incompatible.

After discussing with Marco Clemenci we agreed adding a variable which can override which project lbexec imports is the cleanest solution. This will then be added to the lb-dev-generated CMake configuration.