Remove search for previous pre-commit caches.

These seem to be very large caches.  Restoring an old one and updating 
it results in a cache larger than the maximum allowed cache size: 200MB. 
 
"Cache size of 254757924 bytes is over the 200MB limit, not saving 
cache."
So if the config changes it is best to just take the cache-miss and 
start from scratch.

.github/workflows/build.yml

@@ -20,8 +20,6 @@ jobs:
           path: ~/.cache/pre-commit
           key: "${{ runner.os }}-pre-commit-\
             ${{ hashFiles('**/.pre-commit-config.yaml') }}"
-          restore-keys: |
-            ${{ runner.os }}-pre-commit-
 
       - name: Cache pip test requirements
         uses: actions/cache@v1