updated go installation method.

[bdmorin]

I recently tried this per the readme.

> GO111MODULE=on go get -u github.com/olivere/iterm2-imagetools/cmd/imgcat                                                                  7.8s  Thu May 19 18:23:23 2022
go: go.mod file not found in current directory or any parent directory.
        'go get' is no longer supported outside a module.
        To build and install a command, use 'go install' with a version,
        like 'go install example.com/cmd@latest'
        For more information, see https://golang.org/doc/go-get-install-deprecation
        or run 'go help get' or 'go help install'.
❯ go version                                                                                                                                       go version go1.18.2 darwin/amd64

So I did this, and it worked.

GO111MODULE=on go install  github.com/olivere/iterm2-imagetools/cmd/imgcat@latest