chore: fix postinstall when using tarball (#60443)
The postinstall script was failing In the case when you download [a
zip](https://codeload.github.com/vercel/next.js/zip/refs/heads/canary)
of the repo.
This PR fixes it so that `git config` can fail silently in that case
when the repo is not using git.
- Related to https://github.com/nodejs/citgm/pull/1044
Closes NEXT-2036
