fix: Masquerade as nightly cargo when invoking flycheck with `-Zscript`

crates/rust-analyzer/src/flycheck.rs

diff --git a/crates/rust-analyzer/src/flycheck.rs b/crates/rust-analyzer/src/flycheck.rs
index 233bede..01ac75c 100644
--- a/crates/rust-analyzer/src/flycheck.rs
+++ b/crates/rust-analyzer/src/flycheck.rs
@@ -588,6 +588,7 @@
                     cmd.arg("--manifest-path");
                     cmd.arg(manifest_path);
                     if manifest_path.extension() == Some("rs") {
+                        cmd.env("__CARGO_TEST_CHANNEL_OVERRIDE_DO_NOT_USE_THIS", "nightly");
                         cmd.arg("-Zscript");
                     }
                 }