v4: Update gfortran executor to use OpenBLAS

[mathomp4]

In order to save space, we move from MKL to OpenBLAS with GNU.

[mathomp4]

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[mathomp4]

We cannot do this at the moment. The issue is the LDAS at the moment assumes "If not NAG, use MKL", see GEOS-ESM/GEOSldas_GridComp#158

I'll draft for now so I don't forget.

[mathomp4]

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[mathomp4]

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