patch-package fails when .yarnrc sets yarn-path

Given this `.yarnrc`:

```
yarn-path "./yarn"
```

`patch-package some-package` will fail to generate a patch with this error message:

```
• Creating temporary folder
• Installing … with yarn
internal/modules/cjs/loader.js:968
throw err;
^

Error: Cannot find module '/…/yarn'
at Function.Module._resolveFilename (internal/modules/cjs/loader.js:965:15)
at Function.Module._load (internal/modules/cjs/loader.js:841:27)
at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:71:12)
at internal/main/run_main_module.js:17:47 {
code: 'MODULE_NOT_FOUND',
requireStack: []
}
```