ci: Don't set version in bundle-size artifacts pipeline

[tylerbutler]

We don't want to set versions for the build-bundle-size-artifacts pipeline because the pipeline is special - it runs a client build but doesn't publish anything. Working around this properly in either the pipeline or flub bump is challenging and would create a much bigger change. Since this is the only pipeline that sets this variable to true, we use that to determine whether to set versions.

I could also add a new parameter, but again, that would be a much bigger change and I don't anticipate the parameter being used anywhere else.

[msfluid-bot]

Baseline CI build failed, cannot generate bundle analysis at this time

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Baseline commit: 0a88f5b

Generated by 🚫 dangerJS against a5839e8

[Abe27342]

for near-term posterity--a sample failure motivating this change

[tylerbutler]

Thanks for the review, @Abe27342!