Go 1.13 slightly changed the behavior GO111MODULE=auto [1], which might
accidentally cause the go command to build the module, for instance in
the checkPhase:
[GO111MODULE=auto] activates the module-aware mode of the go command
whenever the current working directory contains, or is below a
directory containing, a go.mod file — even if the current directory
is within GOPATH/src.
[1] https://golang.org/doc/go1.13#proxy-vars
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