fix library doctest: not failing on some feature gates as expected

Updating the behavior of `fn is_unstable_reexport` to return false for bootstrap environment
would result in a breakage of #94972.
This was the quickest and simplest solution to fix this elusive bug, which was only identified
through miri tests.

Signed-off-by: ozkanonur <[email protected]>

Author: onur-ozkan

compiler/rustc_passes/src/stability.rs

@@ -830,6 +830,9 @@ impl<'tcx> Visitor<'tcx> for Checker<'tcx> {
                             None,
                             path.span,
                             None,
+                            #[cfg(bootstrap)]
+                            AllowUnstable::No,
+                            #[cfg(not(bootstrap))]
                             if is_unstable_reexport(self.tcx, id) {
                                 AllowUnstable::Yes
                             } else {