Rollup merge of #42354 - Mark-Simulacrum:reduce-verbosity, r=alexcrichton

Reduce verbosity of build logs

This does two separate things.
 - Sets sccache logging to warn instead of info.
 - Makes tests when running for a given PR (not on auto branch) quiet. (cc @eddyb)

r? @alexcrichton

src/bootstrap/native.rs

@@ -184,7 +184,7 @@ pub fn llvm(build: &Build, target: &str) {
     configure_compilers(&mut cfg);
 
     if env::var_os("SCCACHE_ERROR_LOG").is_some() {
-        cfg.env("RUST_LOG", "sccache=info");
+        cfg.env("RUST_LOG", "sccache=warn");
     }
 
     // FIXME: we don't actually need to build all LLVM tools and all LLVM