set CARGO_INCREMENTAL=0 on CI, it's just a waste of time there

.appveyor.yml

@@ -46,6 +46,7 @@ build_script:
 test_script:
     - set RUST_TEST_NOCAPTURE=1
     - set RUST_BACKTRACE=1
+    - set CARGO_INCREMENTAL=0
     # Test host miri: 32bit Windows
     - cargo miri setup
     - set MIRI_SYSROOT=%USERPROFILE%\AppData\Local\rust-lang\miri\cache\HOST

.travis.yml

@@ -19,6 +19,7 @@ env:
   global:
   - RUST_TEST_NOCAPTURE=1
   - RUST_BACKTRACE=1
+  - CARGO_INCREMENTAL=0
 
 before_script:
 # Compute the rust version we use. We do not use "language: rust" to have more control here.

miri

@@ -49,7 +49,10 @@ fi
 # We enable debug-assertions to get tracing.
 # We enable line-only debuginfo for backtraces.
 export RUSTFLAGS="-C link-args=-Wl,-rpath,$LIBDIR -C debug-assertions -C debuginfo=1 $RUSTC_EXTRA_FLAGS"
-export CARGO_INCREMENTAL=1
+if [ -z "$CARGO_INCREMENTAL" ]; then
+    # Default CARGO_INCREMENTAL to 1.
+    export CARGO_INCREMENTAL=1
+fi
 
 ## Helper functions