ci: build cog binary once and fix goreleaser snapshot detection (#2713)
Build the cog CLI binary in a dedicated build-cog job and share it as
an artifact across integration test shards, instead of rebuilding it
in every shard. This removes Rust toolchain, cargo-binstall, and mise
from the integration shard setup since they were only needed for the
build step.
Also simplify build:cog to always use --snapshot. The git name-rev
check was broken: it resolves to the nearest reachable tag (e.g.
v0.17.0-alpha4~23 from a different branch), takes the --auto-snapshot
path, and goreleaser fails because the tag doesn't point to HEAD. Dev
builds never need tag validation — releases use goreleaser-action
directly.