Remove target/debug/incremental/ before caching on CI

For some reason, incremental files are changing on CI even when no code
changes.  Dropping them from the cache to see if that helps.

Author: jtgeibel

script/ci/prune-cache.sh

@@ -25,5 +25,8 @@ for name in $bin_names; do
     rm -v target/debug/deps/$normalized-*
 done
 
+echo "Removing target/debug/incremental/"
+rm -rf target/debug/incremental/
+
 echo "Final cache size:"
 du -hs target/debug