Bug 1610578 - Add cross sysroot for macOS sccache build. r=froydnj

Depends on D60536

Differential Revision: https://phabricator.services.mozilla.com/D60592

--HG--
extra : moz-landing-system : lando

taskcluster/scripts/misc/build-sccache.sh

@@ -36,6 +36,7 @@ Linux)
         export PATH="$MOZ_FETCHES_DIR/cctools/bin:$PATH"
         export RUSTFLAGS="-C linker=$GECKO_PATH/taskcluster/scripts/misc/osx-cross-linker"
         export CC="$MOZ_FETCHES_DIR/clang/bin/clang"
+        export TARGET_CC="$MOZ_FETCHES_DIR/clang/bin/clang -isysroot $MOZ_FETCHES_DIR/MacOSX10.11.sdk"
         cargo build --features "all" --verbose --release --target $TARGET
     else
         # We can't use the system openssl; see the sad story in