build: use nyc's merge command

a 'merge' command has recently been added to nyc, eliminating the need
for the istanbul-merge library.

Makefile

@@ -165,8 +165,6 @@ coverage: coverage-test ## Run the tests and generate a coverage report.
 .PHONY: coverage-build
 coverage-build: all
 	mkdir -p node_modules
-	if [ ! -d node_modules/istanbul-merge ]; then \
-		$(NODE) ./deps/npm install istanbul-merge --no-save --no-package-lock; fi
 	if [ ! -d node_modules/nyc ]; then \
 		$(NODE) ./deps/npm install nyc --no-save --no-package-lock; fi
 	if [ ! -d gcovr ]; then git clone -b 3.4 --depth=1 \
@@ -195,11 +193,11 @@ coverage-test: coverage-build
 	mv lib lib__
 	mv lib_ lib
 	mkdir -p coverage .cov_tmp
-	$(NODE) ./node_modules/.bin/istanbul-merge --out \
-		.cov_tmp/libcov.json 'out/Release/.coverage/coverage-*.json'
+	$(NODE) ./node_modules/.bin/nyc merge 'out/Release/.coverage' \
+		.cov_tmp/libcov.json
 	(cd lib && .$(NODE) ../node_modules/.bin/nyc report \
 		--temp-directory "$(CURDIR)/.cov_tmp" \
-		--report-dir "../coverage")
+		--report-dir "$(CURDIR)/coverage")
 	-(cd out && "../gcovr/scripts/gcovr" --gcov-exclude='.*deps' \
 		--gcov-exclude='.*usr' -v -r Release/obj.target \
 		--html --html-detail -o ../coverage/cxxcoverage.html \