Fix OpenACC configuration for LUMI (!84)

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## What is the bug
LUMI does not have `OpenACC_C` and the BB fails because we are checking `OpenACC` for both `C` and `Fortran`. This MR fixes `lumi_gpu` BB.
## How do you fix it
Only check if `OpenACC_Fortran` is found.

Approved-by: Jonas Jucker <jonas.jucker@env.ethz.ch>
Merged-by: Jonas Jucker <jonas.jucker@env.ethz.ch>
Changelog: bugfix