Fix shader timestamp churn on checked-in shaders

[gpx1000]

Optimize shader build system to avoid unnecessary rebuilds and source tree modifications

Use copy_if_different instead of copy to prevent timestamp churn on checked-in shader binaries. Change MAIN_DEPENDENCY to DEPENDS and add VERBATIM to custom commands for HLSL, Slang, GLSL, and SPIR-V assembly shader compilation.

This is part of my make the build system better work. This fixes the issue we have called out for it so I'm breaking this change out of the overall change that I'm working on for cmake to improve our build speed on all platforms.

Fixes #1248

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