Rollup merge of #80924 - teryror:issue-80893-fix, r=jyn514

Fix rustdoc --test-builder argument parsing

My suggested fix to issue #80893. I can actually hook Miri in there now.

I also fixed what I believe to be a typo in the option's help text.

src/librustdoc/lib.rs

@@ -412,11 +412,7 @@ fn opts() -> Vec<RustcOptGroup> {
             )
         }),
         unstable("test-builder", |o| {
-            o.optflag(
+            o.optopt("", "test-builder", "The rustc-like binary to use as the test builder", "PATH")
-                "",
-                "test-builder",
-                "specified the rustc-like binary to use as the test builder",
-            )
         }),
         unstable("check", |o| o.optflag("", "check", "Run rustdoc checks")),
     ]

src/test/rustdoc/issue-80893.rs

@@ -0,0 +1,6 @@
+// compile-flags: --test -Z unstable-options --test-builder true
+
+/// ```no_run
+/// This tests that `--test-builder` is accepted as a flag by rustdoc.
+/// ```
+pub struct Foo;