chore(ci): make KEYCAST_BINARY absolute in the e2e GitHub Actions workflow

[NotThatKindOfDrLiz]

PR #173 wires KEYCAST_BINARY=./target/debug/keycast into the new Playwright e2e workflow.

This works today because spawned processes resolve the relative path against the workspace root, but that assumption is subtle and brittle.

Follow-up:

• set KEYCAST_BINARY to an absolute path such as ${{ github.workspace }}/target/debug/keycast
• make sure both the long-running background process and the Playwright-spawned processes use the same binary path without relying on cwd semantics

This is a non-blocking robustness cleanup.

[sergey3bv]

Hey, @NotThatKindOfDrLiz, I opened a PR to solve the issue, could you please take a look?

cc @dcadenas