servo: Merge #19835 - Enable sccache for all builds (from jdm:enable-sccache); r=emilio

Rebase of #17171.

Source-Repo: https://github.com/servo/servo
Source-Revision: 65f549094f4664c5fb1cd8a17dd2cab0ebcf1806

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 33415ab3b5500de962f41a7de5242406ac2e384f

servo/etc/ci/buildbot_steps.yml

@@ -1,6 +1,8 @@
 env:
   RUSTFLAGS: -Dwarnings
   CARGO_INCREMENTAL: "0"
+  CCACHE: sccache
+  RUSTC_WRAPPER: sccache
 
 mac-rel-wpt1:
   - ./mach clean-nightlies --keep 3 --force