The --allow-missing-runtimes options will allow flatpak builder to
not abort immediately if the sdk or runtime for the app being built
are missing.
This option will be useless when building anything in the modules
section of the app manifest. The calls to flatpak-build will fail
because of the missing sdk.
However, it may be useful when an application does not require
building anything inside the sandbox, and the application files
will be installed via other means.
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