don't add cross-compile prefix for CC twice

[benlorenz]

make re-exports changed environment variables to subprocesses, this causes the make subprocess for the src or deps folders to have this applied a second time.

This was observed while trying to build libjulia for Yggdrasil (JuliaPackaging/Yggdrasil#6543) which has CC=cc in the environment, the main make process then changes this to CC=aarch64-apple-darwin20-clang (cc is clang in this case). But the subprocess make -C src will then try to use

CC=aarch64-apple-darwin20-aarch64-apple-darwin20-clang

which fails and then falls back to $(CROSS_COMPILE)gcc which is not what we want to use for this platform.

This assignment was added recently by @vtjnash in #49081
cc: @giordano @fingolfin