Remove npm bin from scripts/gen.sh.

`npm bin` was [removed in npm 9](npm/cli#5459).
`npx` or `npm exec` should be used instead. [This
comment](aws#24217 (comment))
on a similar PR, and the discussion on the [PR in npm removing `npm
bin`](npm/cli#5459) explain why `npx lerna` is
preferable.

scripts/gen.sh

@@ -1,7 +1,6 @@
 #!/bin/bash
 set -euo pipefail
 
-export PATH=$(npm bin):$PATH
 export NODE_OPTIONS="--max-old-space-size=8196 ${NODE_OPTIONS:-}"
 
 echo "============================================================================================="
@@ -15,8 +14,8 @@ fail() {
 
 echo "============================================================================================="
 echo "building required build tools..."
-time lerna run --stream build --scope @aws-cdk/cfn2ts --scope @aws-cdk/ubergen --include-dependencies || fail
+time npx lerna run --stream build --scope @aws-cdk/cfn2ts --scope @aws-cdk/ubergen --include-dependencies || fail
 
 echo "============================================================================================="
 echo "executing gen..."
-time lerna run --stream gen || fail
+time npx lerna run --stream gen || fail