Use $RUSTC instead of rustc to get version if var is available (#15250)

Fixes rust-lang/rust-clippy#15249

For the builtin rustc, it should return "dev" as the release channel.
Clippy tests pass with this patch in both the Clippy standalone
repository and from within the compiler repository.

changelog: none

rustc_tools_util/src/lib.rs

@@ -157,7 +157,8 @@ pub fn get_commit_date() -> Option<String> {
 
 #[must_use]
 pub fn get_compiler_version() -> Option<String> {
-    get_output("rustc", &["-V"])
+    let compiler = std::option_env!("RUSTC").unwrap_or("rustc");
+    get_output(compiler, &["-V"])
 }
 
 #[must_use]
@@ -172,6 +173,8 @@ pub fn get_channel(compiler_version: Option<String>) -> String {
             return String::from("beta");
         } else if rustc_output.contains("nightly") {
             return String::from("nightly");
+        } else if rustc_output.contains("dev") {
+            return String::from("dev");
         }
     }