make os_arch_seen a local variable in sh

release-tools/build.make

@@ -78,11 +78,14 @@ FULL_LDFLAGS = $(LDFLAGS) $(IMPORTPATH_LDFLAGS) $(EXT_LDFLAGS)
 # defined by BUILD_PLATFORMS.
 $(CMDS:%=build-%): build-%: check-go-version-go
 	mkdir -p bin
-	# OS_ARCH_SEEN captures all of the $os-$arch seen for the current binary
+	# os_arch_seen captures all of the $os-$arch seen for the current binary
 	# that we want to build, if we've seen an $os-$arch before it means that
 	# we don't need to build it again, this is done to avoid building
 	# the windows binary multiple times (see the default value of $BUILD_PLATFORMS)
-	export OS_ARCH_SEEN="" && echo '$(BUILD_PLATFORMS)' | tr ';' '\n' | while read -r os arch suffix base_image addon_image; do \
+	echo '$(BUILD_PLATFORMS)' | tr ';' '\n' | while read -r os arch suffix base_image addon_image; do \
+		if ! [ -z $$os_arch_seen ]; then \
+			os_arch_seen=""; \
+		fi; \
 		os_arch_seen_pre=$${OS_ARCH_SEEN%%$$os-$$arch*}; \
 		if ! [ $${#os_arch_seen_pre} = $${#OS_ARCH_SEEN} ]; then \
 			continue; \
@@ -91,7 +94,7 @@ $(CMDS:%=build-%): build-%: check-go-version-go
 			echo "Building $* for GOOS=$$os GOARCH=$$arch failed, see error(s) above."; \
 			exit 1; \
 		fi; \
-		OS_ARCH_SEEN+=";$$os-$$arch"; \
+		os_arch_seen+=";$$os-$$arch"; \
 	done
 
 $(CMDS:%=container-%): container-%: build-%