Fix commit detected when using `download-rustc`

On reflection on the issue in https://github.com/rust-lang/rust/pull/79540#discussion_r572572280,  I think the bug was actually using the `compiler/` filter, not using `--author=bors`. https://github.com/rust-lang/rust/commit/9a1d6174c925f54c923599e29b09d6855e6b3a78 has no CI artifacts because it was merged as part of a rollup:
```
$ curl -I https://ci-artifacts.rust-lang.org/rustc-builds/96e843ce6ae42e0aa519ba45e148269de347fd84/rust-std-nightly-x86_64-unknown-linux-gnu.tar.xz
HTTP/2 404
```
So 9a1d6174c925f54c923599e29b09d6855e6b3a78 is the correct commit to download, and that's what `--author=bors` does:

$ git log --author=bors 4aec8a5da5547d6e1c24e99dad0003b7cac107f5
commit 9a1d6174c925f54c923599e29b09d6855e6b3a78

Ideally it would look for "the most recent bors commit not followed by a change to `compiler/`", which would exclude things like documentation changes and avoid redownloading more than necessary, but
- Redownloading isn't the end of the world,
- That metric is hard to implement, and
- Documentation-only or library-only changes are very rare anyway since they're usually rolled up with changes to the compiler.

0 files changed, 0 insertions, 0 deletions