correct overeager slash conversion on windows

Bazel's File().path method will return forward-slashed paths; this means
if you pass a flag like `"/FI" + f.path` it'll end up looking like
`/FIpath/to/thing`; however, _convert_flags was converting all / to -,
which ended up breaking the path. This makes sure to only replace the
_first_ /.

foreign_cc/private/cc_toolchain_util.bzl

@@ -330,7 +330,7 @@ def _convert_flags(compiler, flags):
         list: The converted flags
     """
     if compiler == "msvc-cl":
-        return [flag.replace("/", "-") if flag.startswith("/") else flag for flag in flags]
+        return [("-" + flag.removeprefix("/")) if flag.startswith("/") else flag for flag in flags]
     return flags
 
 def _add_if_needed(arr, add_arr):