fix: prevent error by running pnpm

[drskur]

When I created infrastructure project using pnpm, the pipeline ran pnpm and the following error occurred.

|[Container] 2023/10/25 07:42:20.717048 Running command npx projen install|
|25|npm WARN exec The following package was not found and will be installed: [email protected]|
|26|/bin/sh: 1: pnpm: not found|
|27|👾 unable to evaluate environment variable PATH=$(pnpm -c exec "node --print process.env.PATH"): unknown error|
|28||
|29|[Container] 2023/10/25 07:42:27.864655 Command did not exit successfully npx projen install exit status 1|
|30|[Container] 2023/10/25 07:42:27.867954 Phase complete: INSTALL State: FAILED|
|31|[Container] 2023/10/25 07:42:27.867971 Phase context status code: COMMAND_EXECUTION_ERROR Message: Error while executing command: npx projen install. Reason: exit status 1|

This patch makes the Synth process take a little longer, but it can fix the problem easily and reliably.