build: let the coverage step's output through, print the report path

Makefile

@@ -131,6 +131,7 @@ test:
 ## Run unit tests under kcov, report in zig-out/coverage/index.html (F= filters)
 coverage:
 	TEST_FILTER="$(or $(F),$(TEST_FILTER))" $(ZIG) build $(ZIGFLAGS) coverage
+	@echo "Coverage report: zig-out/coverage/index.html"
 
 ## Run demo/runner end to end tests
 end2end:

build.zig

@@ -212,6 +212,8 @@ pub fn build(b: *Build) !void {
             .test_runner = .{ .path = b.path("src/test_runner.zig"), .mode = .simple },
         });
         const kcov = b.addSystemCommand(&.{ "kcov", "--clean" });
+        // Otherwise stdio is checked and the runner's stderr summary fails the step.
+        kcov.has_side_effects = true;
         kcov.addPrefixedDirectoryArg("--include-path=", b.path("src"));
         const report = kcov.addOutputDirectoryArg("coverage");
         kcov.addArtifactArg(cov_tests);