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 also fixes a bug with the current
ARB_gl_spirv support.

Fixes: ffdb44d3 ("nir/linker: Add inputs/outputs to the program resource list")

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
(cherry picked from commit 1abca2b3)