Add CI check: build sandbox

[ZhukMax]

Currently only examples are built on CI.

Tasks:

• Add cargo build -p efx-sandbox to GitHub Actions.

• Ensure sandbox compiles with latest code.

Acceptance criteria:
CI guarantees sandbox always compiles.

[aravindan888]

I would like to take this issue, can you assign me

[ZhukMax]

I would like to take this issue, can you assign me

I assigned you, just make fork and make CI check in new branch. Then you can make MR to new release branch (I think, it'll be v0.7)

[ZhukMax]

@aravindan888 hi, I see you made pull request to main branch, please make PR to branch v0.6 and cancel PR #31

[aravindan888]

done, new pr #33 into v0.6, old pr #31 is now closed.

[ZhukMax]

• GitHub Actions now correctly triggers efx-sandbox builds

• cargo build -p efx-sandbox runs without compilation or linking errors on ubuntu-latest.