Verilator 5.x source build in a slim Ubuntu 24.04 image: add libfl-dev, and the install prefix is not relocatable

Building Verilator v5.054 from git in ubuntu:24.04 with apt-get install --no-install-recommends flex bison ... fails at V3ParseLex.cpp with fatal error: FlexLexer.h: No such file or directory. The header lives in libfl-dev, which flex only Recommends, so install libfl-dev explicitly.

Second trap, if you cache the install between CI runs: ./configure --prefix=$DIR bakes that path in. When I restored the same install under a different absolute path, models built with verilator --binary failed with No rule to make target <old prefix>/share/verilator/include/verilated.cpp, even though VERILATOR_ROOT was exported to the new path. Either restore it at the identical path (GitHub-hosted runners keep the workspace path fixed per repository, so an actions/cache of the install works there) or symlink the old prefix to the new location.