Clear out any old toolchains on the builder

Author: bobbobbio

.github/workflows/ci.yml

@@ -42,6 +42,9 @@ jobs:
           ref: bobbobbio/rust-1.88
           path: risc0
 
+      - name: Remove old toolchains
+        run: rm -r $HOME/.risc0/toolchains
+
       - name: Build
         run: GITHUB_ACTIONS=false cargo run --bin rzup -- build rust --path $GITHUB_WORKSPACE/rust
         working-directory: risc0