CI: coverity: simplify debugging options

CI: coverity: simplify debugging options

We used to rely on a call to "sed" to modify the DEBUG option in the
makefile when running under Coverity because it splits words around
spaces and does not allow to pass multi-word build options. As reported
by Tim in issue #1592, this broke with commit 8de7f2822 ("BUILD: makefile:
enable both DEBUG_STRICT and DEBUG_MEMORY_POOLS by default") when the
default DEBUG options changed.

Let's change this to pass all DEBUG options one at a time instead and
get rid of this sed.