os: TestPipeEOF failures with "timed out waiting for read"

[bcmills]

2019-12-12T13:32:03-c39cd41/darwin-amd64-10_14:

--- FAIL: TestPipeEOF (1.08s)
    pipe_test.go:378: line 0
    pipe_test.go:378: line 1
    pipe_test.go:378: line 2
    pipe_test.go:386: timed out waiting for read
FAIL
FAIL	os	2.452s

See previously #35275, which had a very similar failure mode but with an additional file already closed message at the end.

This is the only failure I could find after CL 204800.

CC @ianlancetaylor

[bcmills]

2020-08-17T14:50:17-7ee2622/solaris-amd64-oraclerel
2020-06-18T20:58:26-a07e281/solaris-amd64-oraclerel
2020-02-07T18:06:58-ff091b5/solaris-amd64-oraclerel

[bcmills]

A more thorough history, including a failure in 2021:

greplogs --dashboard -md -l -e 'FAIL: TestPipeEOF .*\n(?: .*\n)* .*: timed out waiting for read'

2021-03-25T19:24:25-569c86d/darwin-amd64-10_14
2020-08-17T14:50:17-7ee2622/solaris-amd64-oraclerel
2020-06-18T20:58:26-a07e281/solaris-amd64-oraclerel
2020-02-07T18:06:58-ff091b5/solaris-amd64-oraclerel
2019-12-12T13:32:03-c39cd41/darwin-amd64-10_14
2019-11-04T10:13:30-1b2ff10/aix-ppc64
2019-11-02T21:51:09-61fa798/aix-ppc64
2019-11-01T00:04:51-4a09a9b/aix-ppc64
2019-10-31T21:47:51-15bff20/windows-amd64-2016
2019-10-31T16:02:25-d5caea7/aix-ppc64