CI: Fix the coverity builds

CI: Fix the coverity builds

In an attempt to fix the use of DEBUG_STRICT commit
7f0f4786d1927f1450392e871480e3122796024e unfortunately broke the Coverity
builds completely.

It turns out that Coverity does not properly handle quoting within
`COVERITY_SCAN_BUILD_COMMAND`, instead breaking up single arguments at
whitespace, thus passing `-DDEBUG_USE_ABORT=1` to `make` as-is.

Fix this issue by hijacking the Makefile within the Coverity workflow. We
simply replace the default value of the `DEBUG` option with whatever values we
need. The build command now only includes the TARGET and USE_* flags, each of
which works without any spaces.