Config is merged and not reloaded on rebuild

[lukaszsamson]

Originally reported in https://elixirforum.com/t/how-can-i-prevent-the-lsp-from-invoking-my-custom-mix-compiler/59802/8?u=lukaszsamson

I put together a minimal reproduction. The bug only seems to occur with a dependency (note that I only tested a path dependency, not git or hex dependencies).

Steps to reproduce:

Clone these repositories into the same directory.
https://git.sr.ht/~garrisonc/exls_bug_config_merge
https://git.sr.ht/~garrisonc/exls_bug_config_merge_parent
Note that the latter has a relative path dependency on the former
Open exls_bug_config_merge_parent/config/config.exs in an LSP-enabled editor.
Note that a warning message has been printed to the LSP log with a dump of the config keys for the child project. This message comes from the Mix compiler task.
Change the config key in the parent’s config.exs to any other value.
Note that the warning message now prints multiple keys, indicating that they have been merged.
Repeat steps 4 and 5 indefinitely.
Tested in neovim on macos via nvim-lspconfig.