Serialize log pauses

[alexey-igrychev]

Concurrent Buildx progress printers call logutil.Pause against one global logrus logger. Pause read and replaced the logger output without serializing the pause intervals, which races under Go's race detector.

Serialize pauses until their buffered output is resumed, and cover concurrent calls with a race regression test.

[alexey-igrychev]

Verification:

• CGO_ENABLED=1 go test -race ./util/logutil passes.
• Mutation: removed the pause mutex → TestPauseConcurrent reports the logrus output race.
• TESTPKGS=./util/logutil TESTFLAGS='-race -run TestPauseConcurrent' make test builds the test image but its test container sets CGO_ENABLED=0, so Go rejects -race.