Add exec-mode to all script files (#1486)

* Add execute mode to all shell scripts

This change adds execute mode (chmod +x)
to add script files in the gcsfuse project.
This is needed to avoid having to
`chmod +x` each time for running a shell
script especially a in jobs or in other scripts.
This `chmod +x` caused errors in running `git checkout`
in the calling script in the scenario like below.

```shell
get checkout master
chmod +x some_child_script.sh
get checkout dev_branch ### FAILS saying files are modified.
```

This change addresses internal bug#307698513 .

* Add execute mode to more shell scripts

* Add exec permission to more script files