Force mythfilldatabase to run from install PREFIX.

This reverts behavior following 6122951 to force the full path
of mythfilldatabase to be absolute, based off the stored install PREFIX,
rather than relying on the binary to show up in the environment's binary
path.

Fixes #11633

mythtv/programs/mythbackend/backendhousekeeper.cpp

@@ -556,7 +556,10 @@ bool MythFillDatabaseTask::DoRun(void)
 
     uint opts = kMSRunShell | kMSAutoCleanup;
     if (mfpath == "mythfilldatabase")
+    {
         opts |= kMSPropagateLogs;
+        mfpath = GetInstallPrefix() + "/bin/mythfilldatabase";
+    }
 
     QString cmd = QString("%1 %2").arg(mfpath).arg(mfarg);