syz-manager: sync saveRepro() and needLocalRepro()

There is a race condition of saving and accessing "repro.prog" file in
processFuzzingResults() goroutine.

needRepro() which is called from reproMgr.Loop() or upon <-externalReproQueue,
or at the end of saveCrash() can be called before saveRepro().

Removed reproMgr.Done channel and moved to post-processing of
*ReproResult to runRepro(). This should guarantee the order of RW
operations on "repro.prog" file.

$ stress ./syz-manager.test -test.run=TestReproRWRace
5s: 16 runs so far, 0 failures, 8 active
10s: 40 runs so far, 0 failures, 8 active
15s: 64 runs so far, 0 failures, 8 active
20s: 80 runs so far, 0 failures, 8 active
25s: 96 runs so far, 0 failures, 8 active
30s: 112 runs so far, 0 failures, 8 active
35s: 136 runs so far, 0 failures, 8 active
40s: 152 runs so far, 0 failures, 8 active
45s: 168 runs so far, 0 failures, 8 active
50s: 184 runs so far, 0 failures, 8 active
55s: 200 runs so far, 0 failures, 8 active
1m0s: 216 runs so far, 0 failures, 8 active

Fixes: https://github.com/google/syzkaller/issues/5157

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