Improve command printing UI in logging

When printing "running commands" in the UI, we display every command line by line, however it is unclear that each command is run in a separate shell, leading to some confusion.

We should clarify and print the commands such that it is clear which line was written in which entry in the YAML (e.g. `build-commands` or `install-commands` entries)

E.g:
```
- ./configure
- cd build
make
```

This was originally filed as a different issue due to this confusion.

#### Original issue for reference

In BuildStream 1, all commands are executed in separate shells in the sandbox's working directory.

In BuildStream 2 when running batch commands (which is default for BuildElement and ScriptElement), commands are batched into a single script, so the `cwd` is preserved across commands.

This should be noted in the porting guide.

Start by locating the logging UI entry point that prints running commands and the porting guide section covering BuildStream 1 and 2 command execution. Trace how YAML command entries are grouped, then make the output identify each entry and document the shell behavior; done when both the UI wording and porting note remove the stated confusion.

Written by the indexing model from the issue text.