Silence some clang-tidy warnings.

- Thread-unsafe std::getenv and std::setlocale doesn't matter for us.
- It is unlikely that we can make main() less complex without making it more
  complex elsewhere.
- Thread-unsafe std::strerror stays unsolved for now.

src/helpers.cpp

@@ -184,7 +184,7 @@ std::string unescape_html(const std::string_view html)
 
 std::string_view get_env(const std::string_view name)
 {
-    const char *env = std::getenv(name.data());
+    const char *env = std::getenv(name.data()); // NOLINT(concurrency-mt-unsafe)
     if (env != nullptr)
     {
         return env;

src/main.cpp

@@ -48,6 +48,7 @@
 
 constexpr int EXIT_FATAL{2}; // NOLINT(readability-identifier-naming)
 
+// NOLINTNEXTLINE(readability-function-cognitive-complexity)
 int main(int argc, char *argv[])
 {
     using namespace epubgrep;
@@ -59,7 +60,7 @@ int main(int argc, char *argv[])
 
     // locale_generator("").name.c_str() returns "*" instead of "". That's why
     // the global C locale isn't changed. So we have to set it additionally.
-    std::setlocale(LC_ALL, "");
+    std::setlocale(LC_ALL, ""); // NOLINT(concurrency-mt-unsafe)
     boost::locale::generator locale_generator;
     locale_generator.add_messages_path("translations");
     locale_generator.add_messages_path("/usr/share/locale");
@@ -174,7 +175,7 @@ int main(int argc, char *argv[])
                 catch (const std::ifstream::failure &e)
                 {
                     LOG(log::sev::error)
-                        << std::strerror(errno)
+                        << std::strerror(errno) // FIXME: Not thread safe.
                         << format(translate(" (while opening {0:s})").str(),
                                   filepath);
                     return EXIT_FAILURE;