fix: Fixed double definition of envvar

[henrique-borba]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Fix compilation error when using CUDA

2.607 ./numpower.c: In function 'zm_deactivate_ndarray':
2.608 ./numpower.c:4104:11: error: redefinition of 'envvar'
2.608      char *envvar = "NDARRAY_VCHECK";
2.608            ^~~~~~
2.608 ./numpower.c:4099:11: note: previous definition of 'envvar' was here
2.608      char *envvar = "NDARRAY_FREEBUFFER";
2.608            ^~~~~~
2.627 make: *** [Makefile:276: install-cuda] Error 1

What is the current behavior? (You can also link to an open issue here)

During compilation with CUDA enabled, the compiler fails because envvar ends up being declared twice.

What is the new behavior (if this is a feature change)?

Change envvar to envvar_vcheck for VCHECK to avoid declaring the variable twice.