fix(ci): set rustup default before rust-cache action

Swatinem/rust-cache runs rustc -vV to compute the cache key, but
rustup has no default toolchain configured in the CI image (mise
manages Rust via shims). Without this, the cache action silently
fails and every run compiles from scratch.

Author: drew

.github/workflows/checks.yml

@@ -34,6 +34,9 @@ jobs:
       - name: Install tools
         run: mise install
 
+      - name: Activate mise environment
+        run: echo "$(mise env --shell bash)" >> "$GITHUB_ENV"
+
       - name: Cache Rust target and registry
         uses: Swatinem/rust-cache@779680da715d629ac1d338a641029a2f4372abb5 # v2
         with: