fix: Public release canary should be using [backends] option (#728)

* Switch to using the [backends] option for public release canary test

Author: kratsg

.github/workflows/release_tests.yml

@@ -25,7 +25,7 @@ jobs:
     - name: Install from PyPI
       run: |
         python -m pip install --upgrade pip setuptools wheel
-        python -m pip install pyhf[tensorflow,torch,minuit,xmlio]
+        python -m pip install pyhf[backends,xmlio]
         python -m pip install 'pytest~=3.5' pytest-cov
         python -m pip list
     - name: Canary test public API