Add make opam-deps-opt to INSTALL.md

[lemmy]

I encountered the following errors after pulling the latest commits from main. I manually installed eio_main with opam install eio_main and lsp with opam install lsp before noticing make opam-deps-opt:

-> % make
dune build
File "lsp/bin/dune", line 7, characters 26-34:
7 |  (libraries tlapm_lsp_lib eio_main cmdliner))
                              ^^^^^^^^
Error: Library "eio_main" not found.
-> required by _build/default/lsp/bin/tlapm_lsp.exe
-> required by alias lsp/bin/all
-> required by alias default
make: *** [build] Error 1

dune build
File "lsp/bin/dune", line 7, characters 12-25:
7 |  (libraries tlapm_lsp_lib eio_main cmdliner))
                ^^^^^^^^^^^^^
Error: Library "tlapm_lsp_lib" in _build/default/lsp/lib is hidden (optional
with unavailable dependencies).
-> required by _build/default/lsp/bin/tlapm_lsp.exe
-> required by alias lsp/bin/all
-> required by alias default
make: *** [build] Error 1