separateDebugInfo: use NIX_RUSTFLAGS

Setting RUSTFLAGS causes Cargo to ignore other ways of configuring
flags, including the target-specific RUSTFLAGS options.  This broke
pkgsCross.musl64.crosvm, and was surprising to users.

Fixes: #261727

pkgs/build-support/setup-hooks/separate-debug-info.sh

@@ -1,7 +1,7 @@
 export NIX_SET_BUILD_ID=1
 export NIX_LDFLAGS+=" --compress-debug-sections=zlib"
 export NIX_CFLAGS_COMPILE+=" -ggdb -Wa,--compress-debug-sections"
-export RUSTFLAGS+=" -g"
+export NIX_RUSTFLAGS+=" -g"
 
 fixupOutputHooks+=(_separateDebugInfo)