test: remove building from source from get prev releases script

Using the get_previous_releases.py script to build from source only works for
releases prior to v29 due to removal of Autotools (in favor of CMake). It also
does not support building on Windows, and we are adding support for downloading
Windows release binaries in later commits of this PR.

As there were no complaints during review, it is assumed nobody uses this
functionality.

ci/test/03_test_script.sh

@@ -99,7 +99,7 @@ if [ -z "$NO_DEPENDS" ]; then
   bash -c "$SHELL_OPTS make $MAKEJOBS -C depends HOST=$HOST $DEP_OPTS LOG=1"
 fi
 if [ "$DOWNLOAD_PREVIOUS_RELEASES" = "true" ]; then
-  test/get_previous_releases.py -b -t "$PREVIOUS_RELEASES_DIR"
+  test/get_previous_releases.py --target-dir "$PREVIOUS_RELEASES_DIR"
 fi
 
 BITCOIN_CONFIG_ALL="-DBUILD_BENCH=ON -DBUILD_FUZZ_BINARY=ON"