Fix issue while sending analytics report

[skshetry]

When sending analytics, we are not flushing the data to the temp file
which causes failure at times, especially on Windows.

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[skshetry]

The test in de21ae4 demonstrates the issue with analytics. See https://github.com/iterative/dvc/actions/runs/1116040914.
Fixed by 9acf0d7.

Here's a snippet of the traceback:

------------------------------ Captured log call -------------------------------
ERROR    dvc:main.py:86 unexpected error
Traceback (most recent call last):
  File "/Users/runner/work/dvc/dvc/dvc/main.py", line 55, in main
    ret = cmd.do_run()
  File "/Users/runner/work/dvc/dvc/dvc/command/base.py", line 64, in do_run
    return self.run()
  File "/Users/runner/work/dvc/dvc/dvc/command/daemon.py", line 32, in run
    analytics.send(self.args.target)
  File "/Users/runner/work/dvc/dvc/dvc/analytics.py", line 67, in send
    report = json.load(fobj)
  File "/Users/runner/hostedtoolcache/Python/3.7.10/x64/lib/python3.7/json/__init__.py", line 296, in load
    parse_constant=parse_constant, object_pairs_hook=object_pairs_hook, **kw)
  File "/Users/runner/hostedtoolcache/Python/3.7.10/x64/lib/python3.7/json/__init__.py", line 348, in loads
    return _default_decoder.decode(s)
  File "/Users/runner/hostedtoolcache/Python/3.7.10/x64/lib/python3.7/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/Users/runner/hostedtoolcache/Python/3.7.10/x64/lib/python3.7/json/decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

This is happening because the file where we are saving the analytics report has been newly created (through NamedTemporaryFile) and we have written some contents to it, but the buffer is not yet flushed.

dvc/dvc/analytics.py

Lines 33 to 35 in 38cd7d0

	with tempfile.NamedTemporaryFile(delete=False, mode="w") as fobj:
	json.dump(report, fobj)
	daemon(["analytics", fobj.name])

[skshetry]

Having a conditional daemonizer would be good for testing and QA, rather than mocking it. But didn't want to introduce big change.

[efiop]

Good catch!