disable RUST_BACKTRACE in CI, set it inside the test harness instead

.github/workflows/ci.yml

@@ -30,7 +30,6 @@ jobs:
             host_target: i686-pc-windows-msvc
     runs-on: ${{ matrix.os }}
     env:
-      RUST_BACKTRACE: 1
       HOST_TARGET: ${{ matrix.host_target }}
     steps:
       - uses: actions/checkout@v3

tests/ui.rs

@@ -125,6 +125,9 @@ fn run_tests(
     // Let the tests know where to store temp files (they might run for a different target, which can make this hard to find).
     config.program.envs.push(("MIRI_TEMP".into(), Some(tmpdir.to_owned().into())));
 
+    // If a test ICEs, we want to see a backtrace.
+    config.program.envs.push(("RUST_BACKTRACE".into(), Some("1".into())));
+
     // Handle command-line arguments.
     let args = ui_test::Args::test()?;
     let default_bless = env::var_os("RUSTC_BLESS").is_some_and(|v| v != "0");
@@ -224,7 +227,7 @@ fn ui(
     with_dependencies: Dependencies,
     tmpdir: &Path,
 ) -> Result<()> {
-    let msg = format!("## Running ui tests in {path} against miri for {target}");
+    let msg = format!("## Running ui tests in {path} for {target}");
     eprintln!("{}", msg.green().bold());
 
     let with_dependencies = match with_dependencies {