chore(scripts): simplify beta release verification steps

[yhabib]

Motivation

The scripts/nns-dapp/release-beta creates an Orbit proposal to release the nns-dapp to Beta. Voters should ideally review the proposal before casting their votes. Currently, the instructions for reviewing the proposal are as follows:

1. git fetch
2. git checkout $COMMIT
3. git merge-base --is-ancestor HEAD origin/main && echo OK || echo \"Commit is not on main branch!\"
4. ./scripts/docker-build
5. sha256sum nns-dapp.wasm.gz"

This is very helpful for understanding what is happening, but it is a bit cumbersome to operate since one has to copy and paste each line.

Changes

• Adds a one-liner with instructions to verify the proposal.

Tests

• Tested locally.

Todos

• Add entry to changelog (if necessary).
  Not necessary.

[mstrasinskis]

Nice!