build: fix yarn errors upon npm install minidump

[Jimbly]

• Only run git submodule update when running out of a git working copy
• Don't require yarn for end-user installation (or any installation, since it's not listed as a dep)

Closes #93

This fixes the following 3 errors:

development without yarn installed globally

$ git clone git@github.com:electron/node-minidump.git && cd node-minidump
$ npm install
> minidump@0.0.0-development preinstall
> yarn submodule && node build.js

sh: 1: yarn: not found

end-user installing of minidump (or any package depending on it) without yarn installed locally or globally

$ mkdir newproj && cd newproj && echo {} > package.json
$ npm i minidump
npm ERR! command failed
npm ERR! command sh -c yarn submodule && node build.js
npm ERR! sh: 1: yarn: not found

end-user installing of minidump (or any package depending on it) in a non-git-controlled working directory:

$ mkdir newproj && cd newproj && echo {} > package.json
$ npm i yarn
$ npm i minidump
npm ERR! command failed
npm ERR! command sh -c yarn submodule && node build.js
npm ERR! yarn run v1.22.22
npm ERR! $ git submodule update --init --recursive
npm ERR! info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
npm ERR! warning ../../package.json: No license field
npm ERR! fatal: not a git repository (or any of the parent directories): .git

Note: simply reverting 4be7737 will solve the last two end-user problems, presumably a situation like the first one was what that commit was attempting to solve (and it did solve it, as long as you have yarn installed globally, which, as far as I know, is not generally recommended), hopefully my addition to build.js will solve more elegantly solve the development case without needing to add external (non-dev) dependencies.

[Jimbly]

Updated (and force-pushed) to remove trailing command that was causing a style check failure.