mkosi: Disable unique debug source names

mkosi: Disable unique debug source names

We use -fdebug-prefix-map= because debugedit doesn't work for us (for
a currently unknown reason since it's the most obtuse code I've ever
had the pleasure of reading). With all the unique macros enabled, the
destination directory we pass to -fdebug-prefix-map= includes the package
release. The release is either the timestamp of the current commit or
the current time if the working tree is dirty. This means it generally
changes every time we rerun the build script. However, meson only reads
compiler arguments the first time it is invoked or if --wipe is specified.
This means that on a rerun -fdebug-prefix-map= will be configured wrong
and the build will fail.

Let's prevent this from happening by disabling the unique debug source
names by overriding the --unique-debug-src-base option that is passed to
find-debuginfo.sh by rpm via the _find_debuginfo_opts macro.