add nightly tests

Author: Christopher Doris

.github/workflows/tests-nightly.yml

@@ -16,7 +16,7 @@ jobs:
       matrix:
         arch: [x64] # x86 unsupported by MicroMamba
         os: [ubuntu-latest, windows-latest, macos-latest]
-        jlversion: ['nightly']
+        jlversion: ['~1.8.0-0', 'nightly']
     steps:
       - uses: actions/checkout@v2
       - uses: julia-actions/setup-julia@v1