fix(rules): Expand array before shell expansion to dedup list

Parameter expansion flags are ignored in here-word syntax, causing
duplicate files to potentially reach the `install` command ... which
objects strenuously.

Follow up question here: https://unix.stackexchange.com/q/694823/1925

rules/utilities.mk

@@ -14,7 +14,7 @@ install-dist: $$(or $$(call extantfiles,$$(DISTFILES)),fail)
 	set -o extendedglob
 	export VERSION_CONTROL=none
 	extants=($(addsuffix ($(hash)qN),$(DISTFILES)))
-	$(XARGS) -r install -m0644 -t "$(DISTDIR)" <<< $${(u)extants}
+	$(XARGS) -r install -m0644 -t "$(DISTDIR)" <<< $${$${(u)extants}}
 
 .PHONY: debug
 debug: