Fix run-local-chain script

Description

Problem: ./run-local-chain fails when MORLEY_LOCALNET is unset due to set -euo.

Solution: use ${parameter:-word} syntax instead.

If parameter is unset or null, the expansion of word is substituted. Otherwise, the value of parameter is substituted.

Related issue(s)

None

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests (see short guidelines)

  • If I added new functionality, I added tests covering it.
  • If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

• Documentation

  • I checked whether I should update the docs and did so if necessary:
    • Root README and other README.md files
    • Haddock
    • docs/
  • I updated changelog files of all affected packages released to Hackage if my changes are externally visible.

Stylistic guide (mandatory)

• My commits comply with the following policy.
• My code complies with the style guide.