rust-common.bbclass: make sure ccache exist

It fails to run task rust_create_wrappers of recipe which inherit
rust-common.bbclass such as rust-hello-world if no host ccache exists:

| DEBUG: Executing shell function do_rust_create_wrappers
| /path/to/project/tmp-glibc/work/core2-64-wrs-linux/rust-hello-world/git-r0/temp/run.do_rust_create_wrappers.3422:
    line 179: ccache: command not found

Run task rust_create_wrappers before do_prepare_recipe_sysroot to make
sure required tools such as ccache exist.

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>