Rename github build workflow

[thockin]

It says amd64, but calls all-build

[justinsb]

Good catch & I really like this. There's a choice in how we name the github actions "jobs", and I have previously proposed mapping them 1:1 to scripts under (for example) dev/ci - with the intention that then it becomes easy to run locally when needed. But if we're using make it makes a lot more sense to align them with the make targets.

(We might want to consider naming the thing which calls make all-build -> all-build, therefore, but now I'm just nit-picking)

/approve
/lgtm

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