curl usage in install script in sync with instructions

[kmt-lnh]

Hi!

This change makes the install script consistent with the installation instructions while keeping wget as an alternative.

Based on the current installation instructions, one starts with invoking curl but the install script gives precedence to wget. It can happen, that curl works on a system, but wget does not. ¹ It is very confusing when the install process starts by downloading the script but then the installer script indicates a network error.

In pull request #5096 @AnatoleLucet and @Mic92 introduced support for wget during the installation. In it Anatole suggested changing the installation instructions from the current curl -L https://nixos.org/nix/install | sh to one that's using wget, but this didn't happen. I simply switched the order of the availability checks to ensure that curl comes first.

Footnotes

1. For example, if the user is behind a proxy and the the environmental variable used is HTTPS_PROXY then curl will work, but wget won't. This is just one example, but as long as curl can work and wgetcan fail these errors can appear and will be confusing. ↩