command/expand: fix data race for concurrent writes

[sonmezonur]

• Concurrent write to objFound bool is not thread-safe. Use atomic boolean to avoid data race.

When I run TestExpandSources with the race detector, it shows data race warnings.

go test -race -mod=vendor -count=1 -run="TestExpandSources" ./...

WARNING: DATA RACE
Write at 0x00c0000b673c by goroutine 30:
  github.com/peak/s5cmd/command.expandSources.func1.1()
      .../go/src/github.com/peak/s5cmd/command/expand.go:79 +0x1d5

Previous write at 0x00c0000b673c by goroutine 29:
  github.com/peak/s5cmd/command.expandSources.func1.1()
      .../go/src/github.com/peak/s5cmd/command/expand.go:79 +0x1d5

Goroutine 30 (running) created at:
  github.com/peak/s5cmd/command.expandSources.func1()
      .../go/src/github.com/peak/s5cmd/command/expand.go:65 +0x1e5

Goroutine 29 (finished) created at:
  github.com/peak/s5cmd/command.expandSources.func1()
      .../go/src/github.com/peak/s5cmd/command/expand.go:65 +0x1e5