ci: improve caching

[shinigami-777]

Description

Issue: #32
This PR improves CI performance by eliminating redundant WASM builds across jobs and caching the Esplora Docker image between runs.
Previously, every Node-related job (build-lint-test-node, esplora-integration, signet-smoke-test) independently ran wasm-pack build --target nodejs --all-features, each taking ~2 minutes. The blockstream/esplora Docker image (~1 GB) was also pulled fresh on every run.

Notes to the reviewers

This is from running the new workflow

Cache entries

Changelog notice

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR