Stop running CI on Windows (#386)

Windows nightlies have been broken for a few days now.

Probably related, core Tensorflow is dropping native Windows support
for anything but tensorflow-cpu (which we don't install by default) as
of the upcoming tf release (2.11).

Both core tensorflow and our development guide recommend WSL2.

I think the simplest solution here is to stop running windows CI and
continue recommending WSL2 for running on windows.

Author: mattdangerw

.github/workflows/actions.yml

@@ -8,10 +8,7 @@ on:
 jobs:
   build:
     name: Run tests
-    runs-on: ${{ matrix.os }}
-    strategy:
-      matrix:
-        os: [windows-latest, ubuntu-latest]
+    runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
       - name: Set up Python 3.7

.github/workflows/nightly.yml

@@ -9,10 +9,7 @@ on:
 jobs:
   build:
     name: Run tests
-    runs-on: ${{ matrix.os }}
-    strategy:
-      matrix:
-        os: [windows-latest, ubuntu-latest]
+    runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
       - name: Set up Python 3.7