Do not download GCC in tests

Author: Kobzol

src/bootstrap/src/core/build_steps/gcc.rs

@@ -93,6 +93,7 @@ pub enum GccBuildStatus {
 /// Tries to download GCC from CI if it is enabled and GCC artifacts
 /// are available for the given target.
 /// Returns a path to the libgccjit.so file.
+#[cfg(not(test))]
 fn try_download_gcc(builder: &Builder<'_>, target: TargetSelection) -> Option<PathBuf> {
     // Try to download GCC from CI if configured and available
     if !matches!(builder.config.gcc_ci_mode, GccCiMode::DownloadFromCi) {
@@ -114,6 +115,11 @@ fn try_download_gcc(builder: &Builder<'_>, target: TargetSelection) -> Option<Pa
     Some(root.join("libgccjit.so"))
 }
 
+#[cfg(test)]
+fn try_download_gcc(_builder: &Builder<'_>, _target: TargetSelection) -> Option<PathBuf> {
+    None
+}
+
 /// This returns information about whether GCC should be built or if it's already built.
 /// It transparently handles downloading GCC from CI if needed.
 ///