When this feature was finally merged, the serenity_option in
lagom_options.cmake had the unintended side effect of always setting the
cache variable to "" in the initial cache. In order to actually set the
linker to use to be lld or mold, we need to set with the FORCE flag in
the use_linker.cmake file.
Found by checking with the CMake variable_watch() function.
This will stop the build from spamming the "Using LLD to link Lagom"
message all over the place :^)
This ports over the `LADYBIRD_USE_LLD` option from the standalone
Ladybird build and generalizes it to work for mold as well: the
`LAGOM_USE_LINKER` variable can be set to the desired name of the
linker. If it's empty, we default to trying LLD and Mold on ELF
platforms (in this order).
