fix miri tests

`cargo miri` started to pick the CARGO env var, nuke that

.github/workflows/ci.yaml

@@ -21,7 +21,7 @@ jobs:
     - uses: actions/checkout@v2
       with:
         fetch-depth: 0 # fetch tags for publish
-
+    - uses: Swatinem/rust-cache@359a70e43a0bb8a13953b04a90f76428b4959bb6
     - run: cargo run -p xtask -- ci
       env:
         CARGO_REGISTRY_TOKEN: ${{ secrets.CRATES_IO_TOKEN }}

xtask/src/main.rs

@@ -11,6 +11,7 @@ fn main() -> xshell::Result<()> {
     let sh = Shell::new()?;
 
     let _e = push_toolchain(&sh, "stable")?;
+    let _e = sh.push_env("CARGO", "");
 
     {
         let _s = section("BUILD");