message: rust: Update minimum toolchain to 1.43.0
diff: prefix ?= usr/local
  BINARY = $(prefix)/bin/ion
  RELEASE = debug
  -TOOLCHAIN ?= 1.39.0
  +TOOLCHAIN ?= 1.43.0
  GIT_REVISION=git_revision.txt
  SRC=Cargo.toml Cargo.lock $(shell find src members -type f -wholename '*src/*.rs')
  