Relax global lock when using describe subcommand

[jchavarri]

Currently if one runs dune build -w in one terminal and tries to run dune describe in another, the latter fails:

$ dune describe
Error: A running dune (pid: 586836) instance has locked the build directory.
If this is not the case, please delete _build/.lock

I understand describe is a "read-only" subcommand, so I wonder if the restrictions / limitations that forced the introduction of the global lock could be relaxed in this case?

Maybe other subcommands that don't write in _build could see this limitation lifted (e.g. dune fmt)

[anmonteiro]

I understand describe is a "read-only" subcommand,

dune describe pp is not read-only, at least, since it builds the ppx pipeline and preprocesses the files.