fix: select medany toolchain and available GDB

source.sh

@@ -29,17 +29,17 @@ done
 export Z3_LIB_DIR="${Z3_LIB_DIR:-$_gps_z3_dir}"
 
 _gps_riscv_gcc=""
-if [[ -n "${TOOLCHAIN:-}" ]]; then
-  _gps_candidates=("$TOOLCHAIN")
-elif [[ -n "${RISCV_TOOLCHAIN:-}" ]]; then
+if [[ -n "${RISCV_TOOLCHAIN:-}" ]]; then
   _gps_candidates=("$RISCV_TOOLCHAIN" "$RISCV_TOOLCHAIN/bin")
+elif [[ -n "${TOOLCHAIN:-}" ]]; then
+  _gps_candidates=("$TOOLCHAIN")
 else
   _gps_candidates=(
     "$GPS_ROOT/tools/local/riscv/bin"
     /tools/toolchains-medany/bin
+    /tools/toolchains/rvtc-medany/bin
     /tools/toolchain/bin
     /tools/toolchains/bin
-    /tools/toolchains/rvtc-medany/bin
   )
 fi