buildtools/wafsamba: use CACHE_SUFFIX instead of a hard-coded name

buildtools/wafsamba: use CACHE_SUFFIX instead of a hard-coded name

waflib.Build provides CACHE_SUFFIX constant to append to the target
name. We have a reference to samba-specific cache suffix (.cache.py)
while original WAF uses _cache.py as a cache suffix since 2011 (see
commit 44a967e326cc2e670a31b3712e4763b72d65e81b in WAF project code).

Signed-off-by: Alexander Bokovoy <ab@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>