hyprland: fix a data race at startup between sockets 1 and 2

`Workspaces::*` and `IPC::startIPC` may both call `getSocketFolder` at the same time.

This randomly causes crashes and/or corruption of the socket path.

Typical crash A:

    [2024-10-16 07:42:09.987] [info] Hyprland IPC starting
    malloc(): unaligned tcache chunk detected
    [2024-10-16 07:42:09.987] [error] Hyprland IPC: Unable to connect?
    Thread 1 "waybar" received signal SIGABRT, Aborted.
    (gdb) bt
    #0  __pthread_kill_implementation (threadid=<optimized out>, signo=signo@entry=6, no_tid=no_tid@entry=0) at pthread_kill.c:44
    (omitted for brievety)
    #9  0x00007ffff64ae745 in operator new (sz=sz@entry=296) at /usr/src/debug/gcc/gcc/libstdc++-v3/libsupc++/new_op.cc:50
    #10 0x00007ffff65ab1f1 in std::filesystem::__cxx11::path::_List::_Impl::copy (this=0x555555a23350) at /usr/src/debug/gcc/gcc/libstdc++-v3/src/c++17/fs_path.cc:249
    #11 0x00007ffff65ab3bd in std::filesystem::__cxx11::path::_List::_List (this=0x7fffffff9d30, other=<optimized out>) at /usr/src/debug/gcc/gcc-build/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/unique_ptr.h:454
    #12 0x00005555556f4ab1 in waybar::modules::hyprland::IPC::getSocket1Reply(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) ()
    #13 0x00005555556f5e3d in waybar::modules::hyprland::IPC::getSocket1JsonReply(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) ()
    #14 0x000055555571289c in waybar::modules::hyprland::Workspaces::setCurrentMonitorId() ()

Typical crash B:

    [2024-10-16 10:01:15.859] [info] Hyprland IPC starting
    [2024-10-16 10:01:15.859] [info] Loading persistent workspaces from Hyprland workspace rules
    Thread 8 "waybar" received signal SIGSEGV, Segmentation fault.
    (gdb) bt
    #0  std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_S_copy
        (__d=0x5555558fbca8 "/", __s=0x2973961a26d35726 <error: Cannot access memory at address 0x2973961a26d35726>, __n=1)
        at /usr/src/debug/gcc/gcc-build/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/basic_string.h:433
    (omitted for brievety)
    #15 waybar::modules::hyprland::IPC::getSocketFolder[abi:cxx11](char const*)
        (instanceSig=0x7fffffffe604 "4520b30d498daca8079365bdb909a8dea38e8d55_1729051218_1982280648") at ../src/modules/hyprland/backend.cpp:41
    #16 0x000055555564230f in waybar::modules::hyprland::IPC::startIPC()::{lambda()#1}::operator()() const ()
        at ../src/modules/hyprland/backend.cpp:70
    #17 0x00007ffff64e1c34 in std::execute_native_thread_routine (__p=0x5555558119c0) at /usr/src/debug/gcc/gcc/libstdc++-v3/src/c++11/thread.cc:104
    #18 0x00007ffff62a339d in start_thread (arg=<optimized out>) at pthread_create.c:447

src/modules/hyprland/backend.cpp

@@ -18,6 +18,9 @@ namespace waybar::modules::hyprland {
 std::filesystem::path IPC::socketFolder_;
 
 std::filesystem::path IPC::getSocketFolder(const char* instanceSig) {
+  static std::mutex folderMutex;
+  std::unique_lock lock(folderMutex);
+
   // socket path, specified by EventManager of Hyprland
   if (!socketFolder_.empty()) {
     return socketFolder_;