fix(sync): declare sort output channels as receive-only <-chan (#835)

[gustcol]

Context

Fixes #835

Root cause

syncObjects declared srcOutputChan and dstOutputChan as
bidirectional chan extsort.SortType. The channels come from
extsort.New(), which the library may return as a receive-only
<-chan extsort.SortType in certain versions. Go's type assignability
rules are strict about channel direction; assigning a receive-only
channel to a bidirectional variable is a compile error:

command/sync.go:372:38: cannot use extsort.New(...) (value of type
  <-chan extsort.SortType) as chan extsort.SortType value in assignment

Fix

Change both variable declarations to <-chan extsort.SortType.
The channels are only ever ranged over (receive operations), so the
narrower type is semantically correct and future-proof against library
changes:

var (
    sorter        *extsort.SortTypeSorter
    srcOutputChan <-chan extsort.SortType   // was: chan extsort.SortType
)

Files changed: command/sync.go (2 lines)

Tests

• Existing command package tests pass.
• The fix is a pure type annotation change; no runtime behavior is affected.
• The build itself on Go 1.23+ is the regression test: if the types were
  wrong, compilation would fail.

Manual verification

go build ./...      # must succeed on Go 1.22 and 1.23+
go test ./command/... # existing tests pass

[gustcol]

Test results

Build (make build): PASS

Unit tests (go test -race ./...): PASS

Wasabi smoke test:

$ ./s5cmd sync /tmp/syncdir/ s3://bucket/sync-test/
cp /tmp/syncdir/test1.txt s3://bucket/sync-test/test1.txt
cp /tmp/syncdir/test2.txt s3://bucket/sync-test/test2.txt
# Exit: 0 — sync runs correctly with receive-only channel types

The <-chan declaration is compatible with the existing code since the channels are only received from (via range), never sent to. Semantically correct and aligns with Go 1.23+ type assignability rules for extsort.New return values.