Unpack bazel in ~/.cache/bazel for better jenkins caching

This might let the bazel server stay up across jobs, speeding the build
up significantly.

Change-Id: I55e5b82061876a64d96d0267936b9d49e9a723ea
diff --git a/tools/bazel b/tools/bazel
index dd5d9a5..a691066 100755
--- a/tools/bazel
+++ b/tools/bazel
@@ -26,7 +26,7 @@
 
 readonly VERSION="0.19.0rc4-201810201638+ac88041"
 
-readonly DOWNLOAD_DIR="$(dirname "${BASH_SOURCE[0]}")/../bazel-downloads"
+readonly DOWNLOAD_DIR="${HOME}/.cache/bazel"
 # Directory to unpack bazel into.  This must change whenever bazel changes.
 readonly VERSION_DIR="${DOWNLOAD_DIR}/${VERSION}-v1"
 readonly VERSION_BAZEL="${VERSION_DIR}/usr/bin/bazel"