guard: a build that cannot run is not a build that refused

TestABuildWithoutTheBuildTagsRefusesAndSaysWhy shells out to the compiler and
treats any error as the refusal it is looking for. A missing compiler arrives
the same way, so on a machine with no Go it reported that the refusal "does
not name build tags" - with the message empty, blaming the refusal rather
than the environment.

Measured on 2026-09-06: that is exactly what it did in the container
tools/linux-check.py uses, which has no Go in it by design. It was the only
test of the whole suite to fail there.

This one cannot use the "an error means skip" shape the other ten callers of
the toolchain use, because here an error is the answer. So it asks
exec.LookPath first, the same idiom and for the same reason as
TestABuildWithNoWindowInItSaysSoAndKeepsStandardOutputEmpty.

Still passes where a compiler exists, and its existing mutation still turns it
red, so the skip did not quietly make it a no-op.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: donislawdev

internal/guard/buildtags_test.go

@@ -147,6 +147,22 @@ func compilesOurCode(line string) bool {
 // the build FAILS. A guard reading internal/format/avif for a build constraint
 // would stay green against a file that had stopped failing.
 func TestABuildWithoutTheBuildTagsRefusesAndSaysWhy(t *testing.T) {
+	// Asked before building, and this one cannot use the "an error means skip"
+	// shape the other ten callers of the toolchain use, because here an error
+	// is the ANSWER. A missing compiler and a compiler that refused both arrive
+	// as err != nil, so without this the test reads "the build failed, which is
+	// right" about a machine that never ran a build, and then reports that the
+	// message does not name build tags - with the message empty.
+	//
+	// Measured on 2026-09-06: that is exactly what it did in the container
+	// tools/linux-check.py uses, which has no Go in it by design. It was the
+	// only test of the suite to fail there, and it blamed the refusal rather
+	// than the environment. Same idiom and same reason as
+	// TestABuildWithNoWindowInItSaysSoAndKeepsStandardOutputEmpty.
+	if _, err := exec.LookPath("go"); err != nil {
+		t.Skipf("no Go toolchain here, so no build can be refused: %v", err)
+	}
+
 	out := filepath.Join(t.TempDir(), "untagged.bin")
 	cmd := exec.Command("go", "build", "-o", out, "./cmd/tfg")
 	cmd.Dir = filepath.Join("..", "..")