Make it possible to only build wasm testcases

[andreaTP]

Why the changes in this PR are needed?

Tackling this "TODO":
https://github.com/open-policy-agent/npm-opa-wasm/blob/92b8aa0234a7e487f2bd335ea1544351718972f7/.github/workflows/ci.yml#L50-L52

as I'm in the same situation:
https://github.com/andreaTP/opa-chicory/blob/d37da558e054510e50da53bf7a1933e0bc19a136/.github/workflows/ci.yml#L33-L34

What are the changes in this PR?

Add the handling for an env var WASM_BUILD_ONLY so that, running:

WASM_BUILD_ONLY=true make wasm-rego-test

Would not run the tests saving time and container images downloads.

Notes to assist PR review:

Attempted to do the least intrusive possible change.

Further comments:

An additional step would be to upload the resulting testcases.tar.gz to GH releases, but I'm looking for feedback before doing the changes.

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[anderseknert]

Hi there, and thanks!

Should you not remove the TODO comment if you resolve that by this fix? :)

The change looks reasonable to me, but I'll let someone else have a say too.

You'll need to sign off on your commit though for the DCO check. You can just do:

git commit --amend --signoff
git push --force

To fix that.

[andreaTP]

Thanks for the review @anderseknert !
The DCO should be fixed now, regarding the TODO, it will require to be merged and used in the SDKs repos to be removed, this PR is just "enabling" a fix down the line.

[anderseknert]

Ah, sorry, I didn't notice that the TODO was in another repo! All good then 👍

[srenatus]

Thanks for your contribution!!

[srenatus]

[nit] I think if we're checking for != "true" below, leaving the default unset would be OK. But it doesn't matter, really.