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

[bcmills]

2021-02-19T00:05:03-0c63312/darwin-amd64-race

--- FAIL: TestFifoEOF (1.09s)
    fifo_test.go:99: line 0
    fifo_test.go:99: line 1
    fifo_test.go:99: line 2
    fifo_test.go:107: timed out waiting for read
FAIL
FAIL	os	12.691s

2020-03-14T07:03:15-d774d97/freebsd-arm-paulzhol

--- FAIL: TestFifoEOF (1.04s)
    fifo_test.go:100: line 0
    fifo_test.go:100: line 1
    fifo_test.go:108: timed out waiting for read
    fifo_test.go:97: read /tmp/workdir-host-freebsd-arm-paulzhol/tmp/TestFifoEOF145353261/fifo: file already closed
    fifo_test.go:86: error closing reader: close /tmp/workdir-host-freebsd-arm-paulzhol/tmp/TestFifoEOF145353261/fifo: file already closed
    fifo_test.go:66: error writing to fifo: write /tmp/workdir-host-freebsd-arm-paulzhol/tmp/TestFifoEOF145353261/fifo: broken pipe
FAIL
FAIL	os	3.573s

Both of these failures occurred after CL 204800, which was believed to fix the problem in #35275 (CC @ianlancetaylor @mknyszek @prattmic).

See also #25877.