Show error details during miri setup in CI

Without this, if it fails, the only information printed is useless:

    Preparing a sysroot for Miri (target: x86_64-unknown-linux-gnu)...
    fatal error: failed to build sysroot; run `cargo miri setup` to see the error details

Author: dtolnay

.github/workflows/ci.yml

@@ -78,6 +78,7 @@ jobs:
     steps:
       - uses: actions/checkout@v3
       - uses: dtolnay/rust-toolchain@miri
+      - run: cargo miri setup
       - run: cargo miri test
         env:
           MIRIFLAGS: -Zmiri-strict-provenance