CI: LDFLAGS for osx coverage

[mwtoews]

This resolves a recent CI issue from macOS that looks like this:

Build coverage as in-source build
...
[ 74%] Linking CXX shared library ../lib/libproj.dylib
Undefined symbols for architecture arm64:
  "_llvm_gcda_emit_arcs", referenced from:
      ___llvm_gcov_writeout in geodesic.c.o
      ___llvm_gcov_writeout in wkt1_generated_parser.c.o
      ___llvm_gcov_writeout in wkt2_generated_parser.c.o
  "_llvm_gcda_emit_function", referenced from:
      ___llvm_gcov_writeout in geodesic.c.o
      ___llvm_gcov_writeout in wkt1_generated_parser.c.o
      ___llvm_gcov_writeout in wkt2_generated_parser.c.o
...

Solution credited to this Q/A.