Fail actions run for nightly failures

Without failing the whole run, it's hard to notice when we get failures in nightly, but nightly should be the advance notice that we pay attention to.

Author: baumanj

.github/workflows/build.yml

@@ -22,15 +22,15 @@ jobs:
           - os: ubuntu-20.04
             rust: nightly
             release: false
-            experimental: true
+            experimental: false
           - os: windows-2019
             rust: nightly
             release: false
-            experimental: true
+            experimental: false
           - os: macos-10.15
             rust: nightly
             release: false
-            experimental: true
+            experimental: false
 
     steps:
     - uses: actions/checkout@v2