glsl/nir: iterate the system values list when adding varyings
Iterate the system values list when adding varyings to the program resource list in the NIR linker. This is needed to avoid CTS regressions when using the NIR to build the GLSL resource list in an upcoming series. Presumably it fixes a bug with current the ARB_gl_spirv support.
Fixes: ffdb44d3 ("nir/linker: Add inputs/outputs to the program resource list")