[core] Follow symlinks in TROOT::GetSharedLibDir()

The idea is to find the actual ROOT install directory, so we have to
follow symlinks. The `info->dlpi_name` doesn't do that, so we need to do
that ourselves with `fs::canonical()`.

Closes #21031.

Author: guitargeek

core/base/src/TROOT.cxx

@@ -3159,7 +3159,18 @@ const TString &TROOT::GetSharedLibDir()
 
       fs::path p = info->dlpi_name;
       if (p.filename() == _R_QUOTEVAL_(LIB_CORE_NAME)) {
-         libdir = p.parent_path().c_str();
+         std::error_code ec;
+
+         // Resolve symlinks: critical for environments like CMSSW, where the
+         // ROOT libraries are loaded via symlinks that point to the actual
+         // install directory
+         fs::path resolved = fs::canonical(p, ec);
+         if (ec) {
+            // Fall back to the loader path if canonicalization fails. The path
+            // will likely be wrong, but at least not garbage
+            resolved = p;
+         }
+         libdir = resolved.parent_path().c_str();
          return 1; // stop iteration
       }
       return 0; // continue