Rollup merge of rust-lang#140394 - Kobzol:git-test-self-contained, r=jieyouxu

Make bootstrap git tests more self-contained

Based on https://stackoverflow.com/a/67512433/1107768.

Fixes: rust-lang#140387

r? ```@jieyouxu```

Author: ChrisDenton

src/bootstrap/src/utils/tests/git.rs

@@ -129,6 +129,9 @@ impl GitCtx {
 
     fn git_cmd(&self) -> Command {
         let mut cmd = Command::new("git");
+        cmd.env("GIT_CONFIG_NOSYSTEM", "1");
+        cmd.env("GIT_CONFIG_SYSTEM", "/tmp/nonexistent");
+        cmd.env("GIT_CONFIG_GLOBAL", "/tmp/nonexistent");
         cmd.current_dir(&self.dir);
         cmd
     }