install,export: Fix export of a genex following $<INSTALL_PREFIX>

The relative path check added by commit 5838aba1aa (Export: Report error
on relative include with genex., 2013-11-26, v3.0.0-rc1~285^2) was added
one condition too early.  If the value starts in `${_IMPORT_PREFIX}`
(which comes from `$<INSTALL_PREFIX>`) then it is an absolute path.

Fixes: #19791

Source/cmExportFileGenerator.cxx

@@ -216,6 +216,9 @@ static bool checkInterfaceDirs(const std::string& prepro,
     if (genexPos == 0) {
       continue;
     }
+    if (cmHasLiteralPrefix(li, "${_IMPORT_PREFIX}")) {
+      continue;
+    }
     MessageType messageType = MessageType::FATAL_ERROR;
     std::ostringstream e;
     if (genexPos != std::string::npos) {
@@ -237,9 +240,6 @@ static bool checkInterfaceDirs(const std::string& prepro,
         hadFatalError = true;
       }
     }
-    if (cmHasLiteralPrefix(li, "${_IMPORT_PREFIX}")) {
-      continue;
-    }
     if (!cmSystemTools::FileIsFullPath(li)) {
       /* clang-format off */
       e << "Target \"" << target->GetName() << "\" " << prop <<