Have rzup put toolchains in a temporary directory

Author: bobbobbio

.github/workflows/ci.yml

@@ -21,6 +21,8 @@ jobs:
             arch: X64
             triple: x86_64-unknown-linux-gnu
     runs-on: [ self-hosted, release, "${{ matrix.os }}", "${{ matrix.arch }}" ]
+    env:
+      RISC0_HOME: $GITHUB_WORKSPACE/rzup_artifacts
     steps:
       - name: Install Rust
         uses: risc0/actions-rs-toolchain@v1
@@ -53,7 +55,7 @@ jobs:
             --exclude lib/rustlib/rustc-src \
             -hczvf \
             ./rust-toolchain-${{ matrix.triple }}.tar.gz \
-            -C $HOME/.risc0/toolchains/*rust*/ \
+            -C $GITHUB_WORKSPACE/rzup_artifacts/toolchains/*rust*/ \
             .
         working-directory: rust