mkosi: Set up -ffile-prefix-map= correctly when building debuginfo packages

mkosi: Set up -ffile-prefix-map= correctly when building debuginfo packages

This makes sure that the debuginfo files contain source files pointing
to the source files shipped by the debugsource package.

Normally this should be done automatically by rpm invoking debugedit
but for some unknown reason debugedit refuses to rewrite the source
files in our binaries.

Given that debugedit is completely undebugable (does not generate any
logs at all, and its source code is ridiculously obtuse), let's set
-ffile-prefix-map= when building instead which achieves the same
effect.