[DEV] Default to bash when running cooker shell. (#108)

When the `SHELL` environment variable is not defined, we have
to default to `/bin/bash` (like when we launch `bitbake`) instead
of `/bin/sh` which is often a link to `/bin/dash`.

Closes: #99

cooker/cooker.py

@@ -636,7 +636,7 @@ def run_bitbake(self, build_config, bb_task, bb_target):
     def shell(self, build_names: List[str]):
         build_dir = self.get_buildable_builds(build_names)[0].dir()
         init_script = self.config.layer_dir(self.distro.BASE_DIRECTORY + "/" + self.distro.BUILD_SCRIPT)
-        shell = os.environ.get('SHELL', '/bin/sh')
+        shell = os.environ.get('SHELL', '/bin/bash')
 
         debug('running interactive, poky-initialized shell {} {} {}', build_dir, init_script, shell)