perf: cache loaded configuration file

[micalevisk]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[x] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: closes #2275

What is the new behavior?

now #load is being invoked only once instead of twice (generate.command, build.action) when running nest build. And only once instead of three times (generate.command, start.action, build.action) on nest start

The downside of this solution is that there's no way to bypass the cache. So a command cannot access a fresh version of the config file unless the CLI process restarts. But AFAIK (and tested) this isn't a feature of the start --watch

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[kamilmysliwiec]

LGTM