guix: Set ZERO_AR_DATE for darwin build determinism

See comments inserted in this commit.

contrib/guix/libexec/build.sh

@@ -148,6 +148,16 @@ export QT_RCC_TEST=1
 export QT_RCC_SOURCE_DATE_OVERRIDE=1
 export TAR_OPTIONS="--owner=0 --group=0 --numeric-owner --mtime='@${SOURCE_DATE_EPOCH}' --sort=name"
 export TZ="UTC"
+case "$HOST" in
+    *darwin*)
+        # cctools AR, unlike GNU binutils AR, does not have a deterministic mode
+        # or a configure flag to enable determinism by default, it only
+        # understands if this env-var is set or not. See:
+        #
+        # https://github.com/tpoechtrager/cctools-port/blob/55562e4073dea0fbfd0b20e0bf69ffe6390c7f97/cctools/ar/archive.c#L334
+        export ZERO_AR_DATE=yes
+        ;;
+esac
 
 ####################
 # Depends Building #