codecov: narrow ext/ ignore to just the CUDA extension

The ignore was added when CUDA became a package extension, to stop the
coverage drop from GPU tests that cannot run on free CI runners. But it
ignored the whole ext/ tree, which also hid the JuMP and Gmsh extensions --
both of which are exercised in CI (test_jump.jl, test_gmsh.jl) and whose
coverage is already in lcov.info. Scope the ignore to
ext/MultiGridBarrierCUDAExt/ so those two extensions report coverage while the
un-runnable CUDA extension stays excluded.

Author: Sébastien Loisel

codecov.yml

@@ -1,3 +1,3 @@
 ignore:
-  - "ext/"
+  - "ext/MultiGridBarrierCUDAExt/"
   - "test/test_cuda.jl"