Fix shared_std_dependency_rebuild running on Windows (#15111)

This fixes the `standard_lib::shared_std_dependency_rebuild` test while
running on Windows. On my system, `CARGO_MANIFEST_DIR` is a normal
windows-style path (`D:\rust\cargo`) with backslashes. That is not valid
TOML syntax. I don't know why this doesn't fail on CI (maybe CI sets a
unix-style current dir?).

tests/testsuite/standard_lib.rs

@@ -264,7 +264,7 @@ fn shared_std_dependency_rebuild() {
                 [build-dependencies]
                 dep_test = {{ path = \"{}/tests/testsuite/mock-std/dep_test\" }}
             ",
-                manifest_dir
+                manifest_dir.replace('\\', "/")
             )
             .as_str(),
         )