cjdns: use temporary directory within the build environment

cjdns: use temporary directory within the build environment

By default, cjdns' build system uses the system wide `/tmp` directory to store
intermediate build artifacts.

Unfortunately its build system fails to clean after itself, leaving thousands
of `jsmake-<hash>` directories behind, taking up precious inodes and massively
slowing down processes traversing the temporary directory:

    root@buildbot:~# find /tmp/ -maxdepth 1 -type d -name 'jsmake-*' | wc -l
    1581

Attempt to solve that problem by using the upstream-introduced
`CJDNS_BUILD_TMPDIR` environment variable in order to move the intermediate
artifacts from the system-wide `/tmp` to a temporary directory within the
package build dir which is properly deleted upon package rebuild cycles.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>