Ref: google#2554

This adds the `cwd` option to the `run spec` command, as asked in #2554.

Since this option is simple and straightforward, I only added a simple formatting rule when retrieving the spec template. I do realize this is not "maintainable" as we add more options, so there are some points that I'd like to raise:

1. I didn't see a lot of tests for cmd files, should I add one for the spec file? If so, any kind of template or good practices I should follow?
2. Other options asked in #2554 are a little more complex since the difference between the default and specified options aren't trivial. I was thinking of creating a full struct specification for OCI configuration files (following [this](https://github.com/opencontainers/runtime-spec/blob/master/config.md)). Any opinions on that matter?

* [x] Have you followed the guidelines in [CONTRIBUTING.md](../blob/master/CONTRIBUTING.md)?
* [x] Have you formatted and linted your code?
* [ ] Have you added relevant tests?
* [x] Have you added appropriate Fixes & Updates references?
* [ ] If yes, please erase all these lines!

---

## Results

```
$ runsc spec --help
spec [options] - create a new OCI bundle specification file.

The spec command creates a new specification file (config.json) for a new OCI bundle.

The specification file is a starter file that runs the "sh" command in the container. You
should edit the file to suit your needs. You can find out more about the format of the
specification file by visiting the OCI runtime spec repository:
https://github.com/opencontainers/runtime-spec/

EXAMPLE:
    $ mkdir -p bundle/rootfs
    $ cd bundle
    $ runsc spec
    $ docker export $(docker create hello-world) | tar -xf - -C rootfs
    $ sed -i 's;"sh";"/hello";' config.json
    $ sudo runsc run hello

  -bundle string
    	path to the root of the OCI bundle (default ".")
  -cwd string
    	working directory that will be set for the executable, this value MUST be an absolute path (default "/")
```

```
$ runsc spec -cwd "/custom/dir"
$ cat config.json
{
	"ociVersion": "1.0.0",
	"process": {
		"terminal": true,
		"user": {
			"uid": 0,
			"gid": 0
		},
		"args": [
			"sh"
		],
		"env": [
			"PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
			"TERM=xterm"
		],
		"cwd": "/custom/dir",
		"capabilities": {
[...]
```

FUTURE_COPYBARA_INTEGRATE_REVIEW=#2767 from mikaelmello:add-cwd-option-spec 9e8000e
PiperOrigin-RevId: 313815101