download-rustc: Don't consider the current commit for download if running in CI

Avoids the following error:
```
curl: (22) The requested URL returned error: 404

error: failed to download pre-built rustc from CI

note: old builds get deleted after a certain time
help: if trying to compile an old commit of rustc, disable `download-rustc` in config.toml:
[rust]
download-rustc = false
```

Note that this strategy is more complicated than the one for download-ci-llvm, which doesn't have to deal with rollup commits.
Eventually we should probably adopt this for LLVM too, to avoid bugs if someone forgets to mark an LLVM bump as rollup=never.

I tested this with `git -c [email protected] -c user.name=bors commit --allow-empty -m 'test bors commit' && GITHUB_ACTIONS=true x check`.

Author: jyn514

src/bootstrap/config.rs

@@ -23,6 +23,7 @@ use crate::channel::{self, GitInfo};
 pub use crate::flags::Subcommand;
 use crate::flags::{Color, Flags, Warnings};
 use crate::util::{exe, output, t};
+use build_helper::ci::CiEnv;
 use build_helper::detail_exit_macro;
 use once_cell::sync::OnceCell;
 use semver::Version;
@@ -1985,11 +1986,15 @@ impl Config {
 
         // Look for a version to compare to based on the current commit.
         // Only commits merged by bors will have CI artifacts.
+
+        // If we are running in CI, the current commit will never have artifacts already built.
+        let tip_commit = if CiEnv::is_ci() { "HEAD^" } else { "HEAD" };
+        // FIXME(#113250): This may not be the best way to find the right commit; it certainly doesn't match what we do for LLVM.
         let merge_base = output(
             self.git()
                 .arg("rev-list")
                 .arg(format!("--author={}", self.stage0_metadata.config.git_merge_commit_email))
-                .args(&["-n1", "--first-parent", "HEAD"]),
+                .args(&["-n1", "--first-parent", tip_commit]),
         );
         let commit = merge_base.trim_end();
         if commit.is_empty() {