OMF's offline installation doesn't really work offline

[w3ntao]

OMF's offline installation tries reaching https://github.com/oh-my-fish during installation, thus fail because of no internet connection.

To reproduce with Docker:

1. prepare a Dockerfile, and have oh-my-fish cloned in the same directory:

$ ls
Dockerfile  oh-my-fish/

$ cat Dockerfile
FROM debian:stable
RUN apt update
RUN apt install -y fish git

COPY oh-my-fish /root/oh-my-fish
WORKDIR /root

2. build docker: docker build . -t debian:omf

3. start docker without network, then proceed OMF offline installation to trigger the error:

$ docker run \
       --network none\
       -it debian:omf fish

# cd oh-my-fish/

# bin/install --offline
Checking for a sane environment...
Installing Oh My Fish to /root/.local/share/omf...
Writing bootstrap to /root/.config/fish/conf.d/omf.fish...
Setting up Oh My Fish configuration...
Updating https://github.com/oh-my-fish/packages-main master... fatal: unable to access 'https://github.com/oh-my-fish/packages-main/': Could not resolve host: github.com
Error
Install aborted: Error installing plugins

Oh My Fish installation failed.

If you think that it's a bug, please open an
issue with the complete installation log here:

http://github.com/oh-my-fish/oh-my-fish/issues

Try rerunning with --verbose to see additional output.

Environment report

Oh My Fish version:   7
OS type:              Debian 11 (docker)
Fish version:         fish, version 3.1.2
Git version:          git version 2.30.2
Git core.autocrlf:    no