test(build): fix data race in TestBuildLayer under -race

[aftersnow]

Summary

Fix a data race in TestBuilderSuite/TestBuildLayer reported by go test -race ./pkg/backend/build/.... Test-only change; production code is untouched.

Root cause

BuildLayer hands OutputLayer a live *io.PipeReader whose backing tar-writer goroutine (archiver.Tar) is mid-write. The subtest uses the generated testify mock for OutputStrategy, and testify's mock.Arguments.Diff formats every actual argument with fmt (fmt.Sprintf("(%[1]T=%[1]v)", actual)) to match the call. Formatting the *io.PipeReader reflects into the pipe's internal sync.Mutex state concurrently with the writer goroutine's Lock() — the reported race.

Write by goroutine (tar): io.(*pipe).write -> sync.(*Mutex).Lock
Read  by goroutine (test): fmt.Sprintf -> reflect.Value.Interface  <- testify Arguments.Diff

No mock matcher avoids it: Diff formats the actual argument before any matcher logic runs, so mock.Anything / AnythingOfType / MatchedBy make no difference.

Fix

Replace the testify mock in the "successful build layer" subtest with a small hand-written OutputStrategy that reads the reader (exactly as the real localOutput/remoteOutput strategies do). This avoids reflecting over the live pipe and unblocks the writer goroutine, while still asserting the call arguments and returned descriptor.

Notes

• Production is unchanged. remoteOutput.OutputLayer relies on the reader's concrete *io.PipeReader type (to drain it when a blob already exists), so wrapping the reader was deliberately avoided.
• Pre-existing: this race is present on main today and is not currently caught by CI (the test targets don't pass -race).

Test plan

• go test -race -count=20 -run TestBuilderSuite/TestBuildLayer ./pkg/backend/build/... — clean (was failing).
• go test -race ./... — all packages clean.
• go vet ./... / gofmt clean.

[gemini-code-assist]

Code Review

This pull request replaces a testify mock with a hand-written recordingStrategy in builder_test.go to resolve a data race caused by concurrent formatting of a live *io.PipeReader. The feedback recommends restoring the original s.builder.strategy at the end of the subtest to prevent test pollution, and initializing the embedded OutputStrategy interface to avoid potential nil pointer dereference panics.

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