Rust Auto Format workflow fails on pull request by other user

[Tamschi]

Describe the bug

The Rust Auto Format workflow fails when running on pull requests made by another user.

Expected behavior

There may be a permission or repository confusion issue here.
The check should be lenient if it can't modify the PR branch.

Screenshots

Error log excerpt:

Fetching the repository
  /usr/bin/git -c protocol.version=2 fetch --no-tags --prune --progress --no-recurse-submodules --depth=1 origin +refs/heads/structopt-for-h*:refs/remotes/origin/structopt-for-h* +refs/tags/structopt-for-h*:refs/tags/structopt-for-h*
  The process '/usr/bin/git' failed with exit code 1
  Waiting 12 seconds before trying again
  /usr/bin/git -c protocol.version=2 fetch --no-tags --prune --progress --no-recurse-submodules --depth=1 origin +refs/heads/structopt-for-h*:refs/remotes/origin/structopt-for-h* +refs/tags/structopt-for-h*:refs/tags/structopt-for-h*
  The process '/usr/bin/git' failed with exit code 1
  Waiting 18 seconds before trying again
  /usr/bin/git -c protocol.version=2 fetch --no-tags --prune --progress --no-recurse-submodules --depth=1 origin +refs/heads/structopt-for-h*:refs/remotes/origin/structopt-for-h* +refs/tags/structopt-for-h*:refs/tags/structopt-for-h*
  Error: The process '/usr/bin/git' failed with exit code 1

please complete the following information:

• rustc --version: Not applicable.
• Crate version (if applicable): Not applicable.

Additional context

See also: Tamschi/reserde#31