ggml-webgpu: avoid using linear_indexing extension in cpy shader

[yomaytk]

Overview

This PR fixes #23925. confirmed that I can correctly run inference on safari.

• reverts to using global_invocation_id for cpy

I introduced global_invocation_index to the cpy shader in #23750 for simplicity, but I noticed it's actually a wgsl extension and safari doesn't seem to support it yet (chrome supports it now).
It looks like we could still use global_invocation_index on chrome by detecting linear_indexing support, but I think global_invocation_id is enough for now.

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: NO

[yomaytk]

@ngxson @reeselevine gentle ping, can either of you check if this fix lets you run it correctly in your environment? :)

[ngxson]

thanks @yomaytk @reeselevine for the quick response, I can confirm that it fixes the problem on safari

[reeselevine]

thanks for the quick fix!