Fix index.json:"platform" in cross-platform builds

[mbargull]

If we cross-build on different platforms and not just different architectures of the same platform, the platform key in index.json is not set to the target platform.
refs:

• https://gitter.im/conda-forge/conda-forge.github.io?at=5faa7d4b8a236947ba9813eb

  I'm having an odd upload issue at https://dev.azure.com/conda-forge/feedstock-builds/_build/results?buildId=237099 .
  The build went fine and uploads for linux-64 and osx-64 were successful. But the win-64 one does not upload (restarted once).
  There is one peculiarity I did: The win-64 one is built on linux-64 -- but that shouldn't cause any issues, shouldn't it? (Given that we upload osx-arm64 packages built from osx-64.)

• https://gitter.im/conda-forge/conda-forge.github.io?at=5faad739c6fe0131d4e912df

  I got it:
  I replaced "platform": "linux", with "platform": "win", in info/index.json and then the upload worked.
  This would also explain why we didn't see this with other cross-compiles before since we had osx-arm64 on osx-64, i.e., unchanged "platform": "osx",. So, conda-build needs a little change to do this then.

[mbargull]

(Adding the actual fix and news entry after seeing the CI fail for the reported issue.)

[mbargull]

=================================== FAILURES ===================================
________________________ test_cross_info_index_platform ________________________
[gw1] darwin -- Python 3.8.3 /Users/runner/work/1/a/miniconda/bin/python
Traceback (most recent call last):
  File "/Users/runner/work/1/s/tests/test_api_render.py", line 173, in test_cross_info_index_platform
    assert metadata.config.host_platform == info_index['platform']
AssertionError: assert 'win' == 'osx'
  - osx
  + win

=================================== FAILURES ===================================
________________________ test_cross_info_index_platform ________________________
[gw0] linux -- Python 3.8.3 /usr/share/miniconda/bin/python
Traceback (most recent call last):
  File "/home/vsts/work/1/s/tests/test_api_render.py", line 173, in test_cross_info_index_platform
    assert metadata.config.host_platform == info_index['platform']
AssertionError: assert 'win' == 'linux'
  - linux
  + win

================================== FAILURES ===================================
_______________________ test_cross_info_index_platform ________________________
[gw0] win32 -- Python 3.8.5 C:\Users\VssAdministrator\Miniconda3\python.exe
Traceback (most recent call last):
  File "D:\a\1\s\tests\test_api_render.py", line 173, in test_cross_info_index_platform
    assert metadata.config.host_platform == info_index['platform']
AssertionError: assert 'linux' == 'win'
  - win
  + linux

🎉 (as intended).

[mbargull]

NB: Of course there are more hurdles for general purpose "Windows on Unix" building (linking, prefix replacement, ...).
The intend from me is to use it for "simple" cases, i.e., only-text-file packages and completely statically linked binaries with no prefix references in them.
refs: conda-forge/containers-common-feedstock#1 , conda-forge/skopeo-feedstock#9

[mbargull]

(Failures on macOS are unrelated, something storage-related or the like.)

[mingwandroid]

Thanks @mbargull

[github-actions]

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