glsl/nir: Don't build soft float64 when it cannot be used
It's not possible to use float64 on GLSL ES, so don't bother trying to build the support code. The support code depends on higher versions of desktop GLSL, so it will fail to compile anyway. This just avoids logging a spurious message.
Fixes: 82d9a37a ("glsl/nir: Add a shared helper for building float64 shaders")
Closes: #5556 (closed)