message: Actually ignore errors
diff: @@ -124,7 +124,9 @@ impl<'cfg> Source for GitSource<'cfg> {
  }
  let git_fs = self.config.git_path();
  - git_fs.create_dir()?;
  + // Ignore errors creating it, in case this is a read-only filesystem:
  + // perhaps the later operations can succeed anyhow.
  + let _ = git_fs.create_dir();
  let git_path = self.config.assert_package_cache_locked(&git_fs);
  // Before getting a checkout, make sure that `<cargo_home>/git` is
  