nir_tests: Add /bigobj when compiling with MSVC
Otherwise the opt_varying tests fail to compile, at least in a debug build.
Fixes: 6dbd1dcd ("nir/tests: add tests for nir_opt_varyings")
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Otherwise the opt_varying tests fail to compile, at least in a debug build.
Fixes: 6dbd1dcd ("nir/tests: add tests for nir_opt_varyings")