Non-hermetic basename tool

[birunts]

A genrule from deb_import.bzl is using non-hermetic basename tool.
This is very simple case here to grab only the file name without any sophisticated usage. Seems to be reasonable to use simple POSIX instead e.g.:

    # gzip compression.

-    data_file="$$(basename $<)"
+    SRC="$<"
+    data_file="$${SRC##*/}"

    if [[ "$$data_file" == "data.tar.bz2" ]]; then

[jjmaestro]

Good catch! Looking at it, I inadvertently added it in #99 but it's not really needed, so I've removed it in #143.

I probably left it when I copy-pasted the script from a standalone version that I wrote in my computer, to test a few thing outside of Bazel before moving it into deb_import 😅