guix: remove ZERO_AR_DATE export

LLD enables ZERO_AR_DATE by default, setting it to zero would enable
non-determinism, setting it to any other value is ignored.

See:
https://github.com/llvm/llvm-project/blob/main/lld/docs/MachO/ld64-vs-lld.rst.

depends/gen_id

@@ -53,7 +53,6 @@
     echo "BEGIN AR"
     bash -c "${AR} --version"
     env | grep '^AR_'
-    echo "ZERO_AR_DATE=${ZERO_AR_DATE}"
     echo "END AR"
 
     echo "BEGIN NM"