feat: add MOZJS_MIRROR and MOZJS_CREATE_MIRROR env variables

[wusyong]

Blocked by #409

This PR adds two env variables for mozjs-sys build script. MOZJS_CREATE_MIRROR can create a spidermonkey tarball for release usage. While MOZJS_MIRROR can use this tarball to extract and link the static libraries without compiling spidermonkey and bindgen wrappers.

[wusyong]

This is the next step to push #439. This is still a draft because #409 hasn't been merged yet. And I haven't decided where to put the tarball yet either.
Right now it can already create the compressed file by setting MOZJS_CREATE_MIRROR=1, and then, reusing it by MOZJS_MIRROR=/path/to/tar.gz.
I'm not sure what github action should be used for releasing it. And I don't have the permission to release either. I'll probably try it with my local fork first to see how it works.

[sagudev]

Why don't we just use uncompressed files instead of taring them, like rusty_v8 is doing?

[wusyong]

Why don't we just use uncompressed files instead of taring them, like rusty_v8 is doing?

rusty_v8 only has one .a static binary AFAIK. They bind the functions manually so they don't have bindgen files.
Do you mean we just upload every single files and download them one by one?
I use tar crate just because it's the first option when I think about grouping the files and extracting them.

[wusyong]

Please let me close this draft since my fork isn't actually forked from this repo which cause a bit inconvenience.
And I would like to test a little bit on my own CI pipeline.
I'll open another one pretty soon.