obey build flags in `garble reverse`

Fixes an unintentional regression in 6ac80db02c.

reverse.go

@@ -35,7 +35,7 @@ One can reverse a captured panic stack trace as follows:
 	pkg, args := args[0], args[1:]
 	// We don't actually run `go list -toolexec=garble`; we only use toolexecCmd
 	// to ensure that sharedCache.ListedPackages is filled.
-	_, err := toolexecCmd("list", []string{pkg})
+	_, err := toolexecCmd("list", append(flags, pkg))
 	defer os.RemoveAll(os.Getenv("GARBLE_SHARED"))
 	if err != nil {
 		return err

testdata/script/reverse.txtar

@@ -43,6 +43,17 @@ cmp stdout reverse.stdout
 stdin main-literals.stderr
 ! exec garble reverse .
 cmp stdout main-literals.stderr
+
+# https://github.com/burrowers/garble/pull/967
+# Ensure that we can reverse a package with build tag required.
+exec garble build -o tag.bin -tags sometag ./case/tag
+exec ./tag.bin
+cp stderr tag.stderr
+! grep "main.CallerFuncName" tag.stderr
+stdin tag.stderr
+exec garble reverse -tags sometag ./case/tag
+stdout main\.CallerFuncName
+
 -- go.mod --
 module test/main
 
@@ -120,6 +131,23 @@ func printStackTrace(w io.Writer) error {
 	return err
 }
 
+-- case/tag/main.go --
+//go:build sometag
+
+package main
+
+import "runtime"
+
+func main() {
+	println(CallerFuncName())
+}
+
+func CallerFuncName() string {
+	pc, _, _, _ := runtime.Caller(0)
+	fn := runtime.FuncForPC(pc)
+	return fn.Name()
+}
+
 -- reverse.stdout --
 lib filename: test/main/lib/long_lib.go