lmms/cmake/linux/apprun-hooks

AppRun Hooks

Scripts placed in this directory will automatically be bundled into linuxdeploy AppImages (e.g. LMMS.AppDir/apprun-hooks) and executed immediately before lmms.

Quoting:

"Sometimes it's important to perform actions before running the actual app. Some plugins might have to set e.g., environment variables to work properly."

See also: https://github.com/linuxdeploy/linuxdeploy/wiki/Plugin-system#apprun-hooks