Use bash in prepare scripts (#339)

Prepare scripts that use `#!/bin/sh` and
`source "$HOME/.sdkman/bin/sdkman-init.sh"`
fail on systems where `sh` is not `bash`,
e.g. on Ubuntu it is `dash` which has not `source`.

prepare_mtopolnik.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 #
 #  Copyright 2023 The original authors
 #