Avoid make-generate downloading packages from the internet

[typeless]

Fixes #7997

[typeless]

~~ Also, I assume the PACKAGES and SOURCES env variables don't need to be configurable. ~~
Skip the related changes.

[lunny]

I think we have -mod=vendor on https://github.com/go-gitea/gitea/blob/master/Makefile#L100 . But it seems this is still necessary.

[typeless]

@lunny yes, but after trying to make generate by myself, I found it does downloads from the internet. The root cause is actually go list ./....

What baffles me right now is that I cannot reproduce the CI error at my local machine.

[typeless]

We still have other dependencies which require downloads such as the swagger tool.
The latest version (go-swagger/go-swagger#1681) of go-swagger has supported modules. Hopefully, we can remove the GO111MODULE=off workaround soon.

[sapk]

@typeless and current go-swagger version require to be in GOPATH which hold us to move to a real mod workflow.

[codecov-io]

Codecov Report

Merging #8008 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #8008      +/-   ##
==========================================
- Coverage    41.6%   41.59%   -0.01%     
==========================================
  Files         479      479              
  Lines       64106    64106              
==========================================
- Hits        26669    26665       -4     
- Misses      33979    33984       +5     
+ Partials     3458     3457       -1

Impacted Files	Coverage Δ
models/unit.go	62.16% <0%> (-5.41%)	⬇️
models/repo_list.go	73.17% <0%> (-0.98%)	⬇️

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