Option to skip-build

[manveru]

While trying to package https://backstage.io/ I ran into a lot of issues related to dependencies that fail to build, most importantly ssh2 requires /usr/bin/env. The package itself isn't really needed by anything I'm using right now afaict, but it requires that I either:

• switch to nodeLinker: pnp, separate ssh2 in its own build and patch it (this doesn't work, because for some reason the typescript stuff goes insane with pnp).
• Set --mode=skip-build on the yarn install step, but there's no clean way to modify options without replacing the whole configurePhase, in which case i lose references to mkCacheBuilderForDrvs, cacheDrvs, yarnPath, and cacheFolder.
• Manually patch the yarn-project.nix after every yarn execution.

So I think the best option would be to offer some way to augment the yarn invocation with additional flags.