compiletest: Use remap-path-prefix only in CI

This makes jump-to-definition work in most IDEs, as well as being easier
to understand for contributors.

src/tools/compiletest/src/header.rs

@@ -6,6 +6,7 @@ use std::io::BufReader;
 use std::path::{Path, PathBuf};
 use std::process::Command;
 
+use build_helper::ci::CiEnv;
 use tracing::*;
 
 use crate::common::{Config, Debugger, FailMode, Mode, PassMode};
@@ -276,8 +277,12 @@ impl TestProps {
     /// `//[foo]`), then the property is ignored unless `cfg` is
     /// `Some("foo")`.
     fn load_from(&mut self, testfile: &Path, cfg: Option<&str>, config: &Config) {
-        // Mode-dependent defaults.
-        self.remap_src_base = config.mode == Mode::Ui && !config.suite.contains("rustdoc");
+        // In CI, we've sometimes encountered non-determinism related to truncating very long paths.
+        // Set a consistent (short) prefix to avoid issues, but only in CI to avoid regressing the
+        // contributor experience.
+        if CiEnv::is_ci() {
+            self.remap_src_base = config.mode == Mode::Ui && !config.suite.contains("rustdoc");
+        }
 
         let mut has_edition = false;
         if !testfile.is_dir() {