Exception on patch create using private registry

The issue is reproduced only with a private registry. Credentials are saved in `~/.npmrc`.
```
❯ npm -v
7.19.0

❯ node -v
v14.17.1

❯ npx patch-package cesium --exclude
patch-package 6.4.7
• Creating temporary folder
• Installing cesium@1.82.1 with npm
{
status: 254,
signal: null,
output: [ null, null, null ],
pid: 27878,
stdout: null,
stderr: null,
error: null
}

.../node_modules/patch-package/dist/makePatch.js:218
throw e;
^
{
status: 254,
signal: null,
output: [ null, null, null ],
pid: 27878,
stdout: null,
stderr: null,
error: null
}
```