Resolve a couple of Github Action CI annoyances: (#4413)

* Set "fail-fast: false" so that multiple jobs in one workflow can
  finish independently. By default, if one job fails, other running jobs
  will be aborted, even if the other jobs are working fine and are
  almost done. This leads to wasted time and resources if the failure
  is, for example, OS specific, or due to a flaky unit test, and the
  failed job needs to be re-run, because all the jobs end up re-running.
* Put conditions back into the windows.yml job (manual, and for
  a specific branch name and that job). This prevents Github Actions
  from sending "No jobs were run" failure emails on every commit.

.github/workflows/nix.yml

@@ -5,6 +5,7 @@ jobs:
 
   test:
     strategy:
+      fail-fast: false
       matrix:
         platform:
           - ubuntu-latest

.github/workflows/windows.yml

@@ -7,11 +7,19 @@ name: windows
 # then, we leave this tombstone as a reminder that we have tried (but failed)
 # to write a reliable test for Windows.
 # on: [push, pull_request]
+on:
+  workflow_dispatch:
+  push:
+    branches:
+      - 'action'
+    paths:
+      - '.github/workflow/windows.yml'
 
 jobs:
 
   test:
     strategy:
+      fail-fast: false
       matrix:
         generator:
           - Visual Studio 16 2019