Allow configuring of GOFLAGS

[Naatan]

I am working on a project that has a vendor dir, but it's only used on certain environments. I want to ignore it during development which is achieved by using GOFLAGS=-mod=mod. The problem is I can't get ZED to respect this.

I've tried setting it as an env var and then running zed, but no dice. I've also tried configuring it via:

  "lsp": {
    "gopls": {
      "initialization_options": {
        "env": {
          "GOFLAGS": "-mod=mod"
        }
      }
    }
  }

Again no dice. In either scenario when I reference symbols that are satisfied so long as you're not using vendor I get: compiler: undefined: symbol.name.