Disable incremental compilation on CI

We don’t keep the `target` directory between builds,
and incremental cache takes a lot of space.

.travis.yml

@@ -40,6 +40,7 @@ matrix:
       env:
         CCACHE=/usr/bin/ccache
         RUSTFLAGS=-Dwarnings
+        CARGO_INCREMENTAL=0
       addons:
         apt:
           sources:

appveyor.yml

@@ -2,6 +2,7 @@ version: 1.0.{build}
 
 environment:
   CCACHE_DIR: "%APPVEYOR_BUILD_FOLDER%\\.ccache"
+  CARGO_INCREMENTAL: 0
   RUST_BACKTRACE: 1
   RUSTFLAGS: -Dwarnings
   # The appveyor image we use has a pretty huge set of things installed... we make the

etc/ci/buildbot_steps.yml

@@ -1,5 +1,6 @@
 env:
   RUSTFLAGS: -Dwarnings
+  CARGO_INCREMENTAL: "0"
 
 mac-rel-wpt1:
   - ./mach clean-nightlies --keep 3 --force