ci: retry/back off toolchain downloads

[gilescope]

Extracted from #442 (the buildkit upgrade). The dominant CI flake class in #442's investigation was network-fetch: toolchain downloads from throttling/dropping CDNs (dl.google.com, ziglang.org, kind.sigs.k8s.io, antlr.org, golangci-lint) die mid-transfer and the dependent build reports a misleading context canceled.

Adds resume + retry/backoff to those downloads:

• root Earthfile — golangci-lint install: curl --retry 7 --retry-all-errors to a temp file, then run it.
• ast/parser/Earthfile — antlr jar: curl -fSLO --retry 7 --retry-all-errors.
• examples/zig/Earthfile — zig tarball: 5-attempt loop with wget --continue + linear backoff (ziglang throttles, so rapid retries hit the same wall).
• tests/cloud-push-pull/Earthfile — gcloud SDK: curl -fO --retry 7 --retry-all-errors.
• tests/with-docker-kind/Earthfile — kind binary: retry loop with wget -c / curl --retry.

Only the download-resilience hunks — no BUILDKIT_PROJECT/image-base/parallelism/group-split changes from the same files (those stay in #442).

[gemini-code-assist]

Code Review

This pull request improves download reliability across multiple Earthfiles by implementing retries, timeouts, and error handling for curl and wget commands. Feedback on these changes includes removing a redundant echo command in ast/parser/Earthfile, using short options for wget in examples/zig/Earthfile to ensure compatibility with minimal environments, and avoiding the use of wget's continue flag (-c) for downloading the kind binary to prevent potential silent corruption.

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[github-actions]

➖ Are we earthbuild yet?

No change in "earthly" occurrences

📈 Overall Progress

Branch	Total Count
main	5328
This PR	5328
Difference	+0

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Keep up the great work migrating from Earthly to Earthbuild! 🚀

💡 Tips for finding more occurrences

Run locally to see detailed breakdown:

./.github/scripts/count-earthly.sh

Note that the goal is not to reach 0.
There is anticipated to be at least some occurences of earthly in the source code due to backwards compatibility with config files and language constructs.

[kmannislands]

It occurs to me that a DOWNLOAD command built in to EarthBuild might be more elegant for this sort of thing long-term