Fix Linux build & some warnings (#6438)

* Fix linux deps debug build

* Use the same DL_CACHE for release build when building debug version of deps on Linux.
This prevents downloading the same source packages twice, and avoid downloading again after deleting the build dir.

* Fix debug build

* Fix warnings "loop variable creates a copy from type" and "loop variable binds to a temporary constructed from type"

src/libslic3r/PrintConfig.cpp

@@ -5329,11 +5329,11 @@ void PrintConfigDef::init_extruder_option_keys()
         "retraction_length",
         "retraction_minimum_travel",
         "retraction_speed",
+        "travel_slope",
         "wipe",
         "wipe_distance",
         "z_hop",
-        "z_hop_types",
-        "travel_slope"
+        "z_hop_types"
     };
     assert(std::is_sorted(m_extruder_retract_keys.begin(), m_extruder_retract_keys.end()));
 }