rustbuild: Pass `ccache` to build scripts

This is a re-attempt at #48192 hopefully this time with 100% less randomly
[blocking builds for 20 minutes][block]. To work around #48192 the sccache
server is started in the `run.sh` script very early on in the compilation
process.

[block]: https://github.com/rust-lang/rust/issues/48192

src/bootstrap/builder.rs

@@ -686,9 +686,25 @@ pub fn cargo(&self,
         //
         // FIXME: the guard against msvc shouldn't need to be here
         if !target.contains("msvc") {
-            let cc = self.cc(target);
-            cargo.env(format!("CC_{}", target), cc)
-                 .env("CC", cc);
+            let ccache = self.config.ccache.as_ref();
+            let ccacheify = |s: &Path| {
+                let ccache = match ccache {
+                    Some(ref s) => s,
+                    None => return s.display().to_string(),
+                };
+                // FIXME: the cc-rs crate only recognizes the literal strings
+                // `ccache` and `sccache` when doing caching compilations, so we
+                // mirror that here. It should probably be fixed upstream to
+                // accept a new env var or otherwise work with custom ccache
+                // vars.
+                match &ccache[..] {
+                    "ccache" | "sccache" => format!("{} {}", ccache, s.display()),
+                    _ => s.display().to_string(),
+                }
+            };
+            let cc = ccacheify(&self.cc(target));
+            cargo.env(format!("CC_{}", target), &cc)
+                 .env("CC", &cc);
 
             let cflags = self.cflags(target).join(" ");
             cargo.env(format!("CFLAGS_{}", target), cflags.clone())
@@ -703,8 +719,9 @@ pub fn cargo(&self,
             }
 
             if let Ok(cxx) = self.cxx(target) {
-                cargo.env(format!("CXX_{}", target), cxx)
-                     .env("CXX", cxx)
+                let cxx = ccacheify(&cxx);
+                cargo.env(format!("CXX_{}", target), &cxx)
+                     .env("CXX", &cxx)
                      .env(format!("CXXFLAGS_{}", target), cflags.clone())
                      .env("CXXFLAGS", cflags);
             }

src/ci/run.sh

@@ -67,6 +67,12 @@ else
   fi
 fi
 
+# We've had problems in the past of shell scripts leaking fds into the sccache
+# server (#48192) which causes Cargo to erroneously think that a build script
+# hasn't finished yet. Try to solve that problem by starting a very long-lived
+# sccache server at the start of the build, but no need to worry if this fails.
+SCCACHE_IDLE_TIMEOUT=10800 sccache --start-server || true
+
 travis_fold start configure
 travis_time_start
 $SRC/configure $RUST_CONFIGURE_ARGS