builddeb: avoid invoking sub-shells where possible

builddeb: avoid invoking sub-shells where possible

The commands surrounded by ( ... ) is run in a sub-shell, but you do
not have to spawn a sub-shell for every single line.

Use just one ( ... ) for creating debian/hdrsrcfiles.

For tar, use -C option instead.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>