[fenix] For https://github.com/mozilla-mobile/fenix/issues/21084: add Runtime.execReadStandardOutOrThrow.
This command makes it a one liner to execute an external command, which will
allow us to easily run git commands in the next section.
Sample error output:
```
* What went wrong:
Execution failed for task ':app:gitHash'.
> command exited with non-zero exit value: 1.
cmd: git reev-parse --short HEAD
stderr:
git: 'reev-parse' is not a git command. See 'git --help'.
The most similar command is
rev-parse
```