xtask-specific binaries in GRAPEVINE_XTASK_PATH

Some possible alternatives to this:

 - Keep putting them in PATH.
 - Make xtask a nix derivation. We would lose out on incremental
   compilation this way, and would end up recompiling xtask from scratch
   whenever something in the main package changed.
 - Have xtask call `nix build --inputs-from $toplevel nixpkgs#go` and
   such. Slow and tedious.

xtask/src/complement/test2json.rs

@@ -45,8 +45,6 @@ pub(crate) fn run_complement(
     test_count: u64,
 ) -> Result<TestResults> {
     // TODO: handle SIG{INT,TERM}
-    // TODO: XTASK_PATH variable, so that we don't need to pollute devshell with
-    // go
     let cmd = cmd!(sh, "go tool test2json complement.test -test.v=test2json")
         .env("COMPLEMENT_BASE_IMAGE", docker_image)
         .env("COMPLEMENT_SPAWN_HS_TIMEOUT", "5")