[core] Warn if auto-determined RootSys differs from ROOTSYS env var

Example of how the warning looks like if it happens:
```txt
[I] omen nix-shell ~/c/r/root_build > ROOTSYS=/home/rembserj/ ./bin/root
Warning in <TROOT>: ROOTSYS is set but inconsistent with detected ROOT installation:
   ROOTSYS=/home/rembserj/
   Detected=/home/rembserj/code/root/root_build
ROOT will use the detected installation.
   ------------------------------------------------------------------
  | Welcome to ROOT 6.41.01                        https://root.cern |
  | (c) 1995-2025, The ROOT Team; conception: R. Brun, F. Rademakers |
  | Built for linuxx8664gcc on Jan 01 1980, 00:00:00                 |
  | From heads/ignoreprefix@v6-99-99-368-gd1d33ba04be                |
  | With clang version 21.1.8 std202002                              |
  | Try '.help'/'.?', '.demo', '.license', '.credits', '.quit'/'.q'  |
   ------------------------------------------------------------------

root [0]
```

Author: guitargeek

core/base/src/TROOT.cxx

@@ -783,6 +783,40 @@ Int_t gDebug;
 
 TROOT::TROOT() : TDirectory() {}
 
+namespace {
+
+void WarnIfInconsistentRootSys()
+{
+   namespace fs = std::filesystem;
+
+   const char *envRootSys = std::getenv("ROOTSYS");
+   if (!envRootSys) {
+      return;
+   }
+   bool isConsistent = false;
+   // Should never throw because it's a valid path
+   fs::path detected = fs::canonical(TROOT::GetRootSys().Data());
+   try {
+      fs::path fromEnv = fs::canonical(envRootSys);
+      if (detected == fromEnv) {
+         isConsistent = true;
+      }
+   } catch (const fs::filesystem_error &) {
+      // Happens if the path in ROOTSYS fails to canonicalize, e.g. because the
+      // directory doesn't exist. In that case, we also print the warning.
+   }
+   if (isConsistent)
+      return;
+   Warning("TROOT",
+           "ROOTSYS is set but inconsistent with detected ROOT installation:\n"
+           "   ROOTSYS=%s\n"
+           "   Detected=%s\n"
+           "ROOT will use the detected installation.",
+           envRootSys, detected.string().c_str());
+}
+
+} // namespace
+
 ////////////////////////////////////////////////////////////////////////////////
 /// Initialize the ROOT system. The creation of the TROOT object initializes
 /// the ROOT system. It must be the first ROOT related action that is
@@ -839,6 +873,10 @@ TROOT::TROOT(const char *name, const char *title, VoidFuncPtr_t *initfunc) : TDi
    GetIconPath();
    GetTTFFontDir();
 
+   // Warn if ROOTSYS is in the environment, but it's inconsistent with what
+   // TROOT has figured out.
+   WarnIfInconsistentRootSys();
+
    gRootDir = GetRootSys().Data();
 
    TDirectory::BuildDirectory(nullptr, nullptr);