Root Cause

The PR diff inserts a second preparation block after startStacks, declaring sshAuth and repoArchive again and passing cacheDir to r.prepare. On 9.5, prepare has the signature prepare(context.Context), and the existing call at pkg/testing/runner/runner.go:133-139 already performs preparation before provisioning. The backport therefore produces both redeclaration errors and an invalid call.

Evidence

• Build: https://buildkite.com/elastic/elastic-agent/builds/42818
• Job: check-ci
• Key log excerpt: pkg/testing/runner/runner.go:149:6: sshAuth redeclared, runner.go:150:6: repoArchive redeclared, runner.go:153:53: undefined: cacheDir, and too many arguments in call to r.prepare (the existing signature accepts only context.Context).
• The same compiler error appears in the Ubuntu, requirefips, fips140-only, amd64/arm64 package, container, and cross-package logs; failures occur before tests or artifacts are produced.
• No matching flaky-test issue was found.

Verification

Not run locally; diagnosis is based on the Buildkite logs, PR diff, and the 9.5 source implementation.