Makefile: Unset GITEA_CUSTOM when running tests

The test code expects that GITEA_CUSTOM is not set. While there is no
reason to set this variable some misguided packagers set it globally in
/etc/profile rather than in the service definition.

Fixes: #36042

Author: hramrach

Makefile

@@ -432,7 +432,7 @@ test: test-frontend test-backend ## test everything
 .PHONY: test-backend
 test-backend: ## test backend files
 	@echo "Running go test with $(GOTESTFLAGS) -tags '$(TEST_TAGS)'..."
-	@$(GO) test $(GOTESTFLAGS) -tags='$(TEST_TAGS)' $(GO_TEST_PACKAGES)
+	@GITEA_CUSTOM= $(GO) test $(GOTESTFLAGS) -tags='$(TEST_TAGS)' $(GO_TEST_PACKAGES)
 
 .PHONY: test-frontend
 test-frontend: node_modules ## test frontend files
@@ -453,7 +453,7 @@ test-check:
 .PHONY: test\#%
 test\#%:
 	@echo "Running go test with -tags '$(TEST_TAGS)'..."
-	@$(GO) test $(GOTESTFLAGS) -tags='$(TEST_TAGS)' -run $(subst .,/,$*) $(GO_TEST_PACKAGES)
+	@GITEA_CUSTOM= $(GO) test $(GOTESTFLAGS) -tags='$(TEST_TAGS)' -run $(subst .,/,$*) $(GO_TEST_PACKAGES)
 
 .PHONY: coverage
 coverage: