Set convert unit to big endian for FastScape

[anne-glerum]

This PR sets an environment variable in the Dockerfile from which the dealii-master tester image is built. The GFORTRAN_CONVERT_UNIT setting is needed to get correct VTU output from FastScape (see fastscape-lem/fastscapelib-fortran#49). Without it, the output is not readible in ParaView. Although this is not so important for the tester, I would like to include it for completion and in case someone uses the image. Also, the FastScape tests do produce VTU output, but I don't think that is compared.

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For new features/models or changes of existing features:

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[gassmoeller]

Yes, makes sense. It is also a good reminder for when we include Fastscape into the official ASPECT docker image, that we need to set this variable.

[gassmoeller]

/rebuild