build: pin Fortio native compatibility fix

[krystophny]

Summary

• pin Fortio to the unique squash-merge commit from Match native HDF5 and NetCDF read compatibility lazy-fortran/fortio#70
• remove the obsolete Fortio-specific workflow_dispatch override
• keep libneo's normal dependency and ordinary test workflow
• do not bump the Fortio version

Fortio's low-level HDF5 and NetCDF compatibility adapters now follow the native libraries' numeric-conversion behavior, while its higher-level typed API remains strict. The merged commit is 6841431f0623083d6ae1ae392abfe8ea3f33fd8a.

Verification

• libneo's ordinary test suite: 107/107 passed
• the same ordinary suite passed in Fortio's provider-independent downstream check
• libneo now contains no Fortio-specific CI test hook; Fortio injects its candidate only in Fortio's own detached checkout

This PR is intentionally left open for downstream review.

[GeorgGrassler]

@krystophny two things:

(a) I am not quite sure about the nature of this pr: Is the intention to run e.g. NEO-2 with this libneo branch checked out to in our daily workload? Or to stay open until the multistage tests are implemented in NEO-2 that check the use case that initially triggered lazy-fortran/fortio#70?

(b) I checked lazy-fortran/fortio#70: I do not see what change there makes the the obsolete Fortio-specific workflow_dispatch override now obsolete. Can you elaborate? Is this a remnant of an older change?