chore(scripts): add missing execa import

See 4dde525996

scripts/release/steps/update-changelog.js

@@ -2,6 +2,7 @@
 
 const chalk = require("chalk");
 const dedent = require("dedent");
+const execa = require("execa");
 const fs = require("fs");
 const prsMergedSince = require("prs-merged-since");
 const semver = require("semver");