[Github] Bump max ccache size for premerge

This patch bumps the max ccache size for premerge jobs to 2GB. Doing a manual
invocation against changes in LLVM produces about 1GB of cache. The default
is 500MB, which we feel might be contributing to the very poor cache
utilization.

Author: boomanaiden154

.github/workflows/premerge.yaml

@@ -30,6 +30,8 @@ jobs:
           fetch-depth: 2
       - name: Setup ccache
         uses: hendrikmuhs/[email protected]
+        with:
+          max-size: "2000M"
       - name: Build and Test
         # Mark the job as a success even if the step fails so that people do
         # not get notified while the new premerge pipeline is in an