GitHub Actions: really use x86 Python on Windows

In a prior commit, I mistakenly used "architecture: x64" instead of
"architecture: x86", and tests actually passed.

That was surprising, because netifaces do not have amd64_win will on
PyPI.  But mystery was solved when itamarst pointed out that netifaces
wheel (that we previously built) is present on pip cache.

But pip cache might be purged one day, and tests will fail again that
day.  We can't have that, so we will try to stick with x86 for now.

.github/workflows/ci.yml

@@ -49,7 +49,7 @@ jobs:
         uses: actions/setup-python@v1
         with:
           python-version: ${{ matrix.python-version }}
-          architecture: x64
+          architecture: x86
 
       # To use pip caching with GitHub Actions in an OS-independent
       # manner, we need `pip cache dir` command, which became
@@ -198,7 +198,7 @@ jobs:
         uses: actions/setup-python@v1
         with:
           python-version: ${{ matrix.python-version }}
-          architecture: x64
+          architecture: x86
 
       - name: Get pip cache directory
         id: pip-cache
@@ -262,7 +262,7 @@ jobs:
         uses: actions/setup-python@v1
         with:
           python-version: ${{ matrix.python-version }}
-          architecture: x64
+          architecture: x86
 
       - name: Get pip cache directory
         id: pip-cache