testing/dotnet6-runtime: set ldpath-recursive in options

Due to changes introduced in !73656 (merged), dotnet8-runtime-bootstrap shared objects scanning was fixed, and thus libmscordaccore.so started being detected as being provided by it. This fix confused dotnet6-runtime dependency scanning as its own libmscordaccore.so was not detected due to not being under /usr/lib.

Thus, ldpath-recursive was added in options to recursively scan for shared objects. A more targetted approach would've been to use: ldpath=/usr/lib/dotnet/shared/Microsoft.NETCore.App/$pkgver

The ldpath-recursive approach was privileged as ldpath affected all (sub)packages globally, and we wanted to also scan shared objects provided by all packages.