Fix Android CI: restore PATH after mise-action stopped leaking runner PATH

[ofalvai]

Summary

• The Android CI job's Run Android Tests step set PATH: .../emulator:.../platform-tools:${{ env.PATH }}, relying on ${{ env.PATH }} (GitHub Actions' env context) to hold the runner's real $PATH.
• That only ever worked because jdx/mise-action had a bug (jdx/mise-action#555) that leaked the entire runner PATH into GITHUB_ENV. That bug was intentionally fixed in mise-action v4.2.1, and since this workflow pins the floating @v4 tag, CI runs after that release started resolving env.PATH to empty — collapsing the step's PATH down to just the two SDK dirs and wiping out /bin, causing Unable to locate executable file: sh.
• This wasn't a regression in this repo's history (rebuilding older commits fails too, since @v4 re-resolves independent of the checked-out commit).

Changes

• Replace the broken env: PATH: ...${{ env.PATH }} override with a dedicated step that appends the Android SDK emulator/platform-tools dirs to $GITHUB_PATH, which correctly extends the real PATH regardless of what mise-action does with GITHUB_ENV.

Test plan

• Confirm the android-test job in CI passes on this PR