CI: fix golangci-lint v2.12.2 update

[Copilot]

Updating GOLINT_VERSION to v2.12.2 failed in CI because the upstream installer no longer verified the correct checksum entry for the release tarball. This updates minikube's install path for golangci-lint and clears the new govet findings introduced by v2.12.2.

• Installer path

  • Replace the install.sh pipe in Makefile with direct download of the release tarball and checksums file.
  • Verify the archive against the checksum entry matched by exact filename, then extract the binary into out/linters.

• Lint compatibility

  • Replace the small set of ioutil calls now flagged by govet with the equivalent os/io APIs in pkg/libmachine/....
  • Keep behavior unchanged while making the tree clean under golangci-lint v2.12.2.

• Key change

  want=$$(awk -v file="$${name}.tar.gz" '$$2 == file { print $$1; exit }' "$${checksums}")
  got=$$(openssl sha256 "$${archive}" | awk '{print $$2}')
  test "$${want}" = "$${got}"

[k8s-ci-robot]

Hi @Copilot. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work.

Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[minikube-bot]

Can one of the admins verify this patch?

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Copilot
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