chore(build): Update for rustc nightly

Author: berkus

Makefile.toml

@@ -70,8 +70,8 @@ VOLUME = { value = "/Volumes/BOOT", condition = { env_not_set = ["VOLUME"] } }
 # Flows that start directly in the member crate, must manually extend the workspace
 # level makefile using the `extend` keyword.
 CARGO_MAKE_EXTEND_WORKSPACE_MAKEFILE = true
-# Build rust stdlib locally
-RUST_STD = "-Zbuild-std=compiler_builtins,core,alloc -Zbuild-std-features=compiler-builtins-mem"
+# Build rust stdlib locally (TODO: expand this var to collectively include other nightly flags)
+RUST_STD = "-Zjson-target-spec -Zbuild-std=compiler_builtins,core,alloc -Zbuild-std-features=compiler-builtins-mem"
 TARGET_JSON = "${CARGO_MAKE_WORKSPACE_WORKING_DIRECTORY}/targets/${TARGET}.json"
 DEVICE_FEATURES = "noserial"
 QEMU_FEATURES = "qemu"