Create ~/.cache/bazel if it doesn't exist

On a fresh jessie install bazel kept failing with the following error:

  $ bazel info
  Downloading Bazel version 201711232100+e0fe5d9 from http://frc971.org/Build-Dependencies/bazel_201711232100+e0fe5d9-linux-x86_64.deb...
  mktemp: failed to create directory via template ‘/home/user/.cache/bazel/tmp.XXXXXXXXXX’: No such file or directory

After creating the parent folders, bazel could complete downloading
and start up.

Change-Id: I9d181328a8bf1159b2b2ac7f7c99aa2fc8ce01b0
diff --git a/tools/bazel b/tools/bazel
index 75faa0f..09e1cb5 100755
--- a/tools/bazel
+++ b/tools/bazel
@@ -50,6 +50,7 @@
   # destination, which is important so we can atomically move it.
   # If this move is non-atomic, then a concurrent Bazel command (like the verifier
   # uses several of) could use a half-copied Bazel installation.
+  mkdir -p "${HOME}/.cache/bazel"
   DOWNLOAD_TEMP_DIR="$(mktemp --directory --tmpdir="${HOME}/.cache/bazel")"
   TEMP_DIR="$(mktemp --directory --tmpdir="${DOWNLOAD_DIR}")"
   readonly DOWNLOAD_TEMP_DIR