oss-fuzz: Build in subdirectory, remove built files
Commit 8183a658 tried to use Meson's "--reconfigure" flag to avoid having to remove all files in "$WORK". Unfortunately doing so had a consequence not noticed at the time: Changed build options no longer cause a rebuild. When selecting another fuzzing engine linker errors would ensue.
With this change the work of commit 8183a658 is partially undone. The build is now happening in a subdirectory of "$WORK", leaving OSS-Fuzz' own content alone, and it's once again fine to remove all object files, libraries and binary built by Meson.