make: pass TESTFLAGS to roachprod-stress, not GOFLAGS

Passing the testflags through the GOFLAGS env var was causing the following
error:
```
stringer -output=pkg/sql/opt/rule_name_string.go -type=RuleName pkg/sql/opt/rule_name.go pkg/sql/opt/rule_name.og.go
stringer: go [list -f {{context.GOARCH}} {{context.Compiler}} -tags= -- unsafe]: exit status 1: go: parsing $GOFLAGS: non-flag "storage.test"
Makefile:1496: recipe for target 'pkg/sql/opt/rule_name_string.go' failed
make: *** [pkg/sql/opt/rule_name_string.go] Error 1
make: *** Waiting for unfinished jobs....
```

Release note: None

Makefile

@@ -973,7 +973,7 @@ stress stressrace:
 roachprod-stress roachprod-stressrace: bin/roachprod-stress
 	# The bootstrap target creates, among other things, ./bin/stress.
 	build/builder.sh make bin/.bootstrap
-	build/builder.sh make test GOFLAGS="$(GOFLAGS) -v -c -o $(notdir $(patsubst %/,%,$(PKG))).test" PKG=$(PKG)
+	build/builder.sh make test GOFLAGS="$(GOFLAGS)" TESTFLAGS="-v -c -o $(notdir $(patsubst %/,%,$(PKG))).test" PKG=$(PKG)
 	@if [ -z "$(CLUSTER)" ]; then \
 	  echo "ERROR: missing or empty CLUSTER"; \
 	else \