Cannot access CodeQL environment variables in `database create --command`

I would like to explicitly invoke the JavaScript autobuilder as part of `codeql database create`, with the ultimate goal of passing it some custom options. However, even invoking it without any options isn't as easy as I would have hoped, since I cannot figure out how to access CodeQL environment variables to specify the path to the autobuilder.

My first attempt of

```sh
codeql database create -l javascript --command '${CODEQL_EXTRACTOR_JAVASCRIPT_ROOT}/tools/autobuild.sh /path/to/database
```

was met with

```sh
A fatal error occurred: Failed to expand '${CODEQL_EXTRACTOR_JAVASCRIPT_ROOT}/tools/autobuild.sh' as an argument list.
(eventual cause: UserError "Attempting to expand unknown variable: CODEQL_EXTRACTOR_JAVASCRIPT_ROOT, available variables are: []")
```

@lcartey suggested adding `env.` like so:

```sh
codeql database create -l javascript --command '${env.CODEQL_EXTRACTOR_JAVASCRIPT_ROOT}/tools/autobuild.sh' /path/to/database
```

but that resulted in

```
A fatal error occurred: Failed to expand '${env.CODEQL_EXTRACTOR_JAVASCRIPT_ROOT}/tools/autobuild.sh' as an argument list.
(eventual cause: UserError "Attempting to expand unknown variable: env.CODEQL_EXTRACTOR_JAVASCRIPT_ROOT, ava...")
```

I am pretty sure this is the right environment variable name, cf. [here](https://git.semmle.com/Semmle/code/blob/master/language-packs/javascript/tools/autobuild.sh#L28).

Specifying the full path to the autobuild script is, of course, possible, but annoying.

Start with the `codeql database create --command` entry point and its argument-variable expansion, using the reported `CODEQL_EXTRACTOR_JAVASCRIPT_ROOT` and `env.` examples as reproduction cases. Trace why the command environment is unavailable during expansion; done when the JavaScript autobuilder path can be supplied through the CodeQL environment variable and the behavior is covered by an appropriate test.

Written by the indexing model from the issue text.